This is a major issue the LessWrong team has thought about a lot. Building off ESRogs' and Elo's answers, the two main reasons are:

1) Privacy, comfort, and filtering.

2) Google Docs has great features and the best collaborative exobrain product.

Privacy/Comfort/Filtering

I think google docs ends up having the best overall conversation, but there is even a marked difference in feel between public LessWrong discussion and, say, Facebook discussion (holding participants constant).

Part of this is privacy concerns like ESRogs notes. If you represent an org maintaining a reputation, or you are discussing infohazardy material, or you have semi-secret information, you simply can't chat freely in public – you have to think carefully about the ramifications of sharing your ideas. This makes sharing ideas harder, and sometimes unpleasant, so people end up doing it less. (semi-private venues like facebook, and many google docs, obviously carry at least some risk, but the risk feels a lot less pronounced).

But I think for a lot of documents, it's less about literal secrecy or organizational PR, and more about an overall sense of safety/trust/filtering. In private you don't have to worry as much about being wrong.

And, to some degree, "fear of being wrong" is something worth getting over. But there are practical matters that mean public discussion will always have some downsides.

In a public space, if you make an off-the-cuff remark that's not fully defensible, you'll often get people critiquing you who miss the point of what you're actually trying to say, or (worse) are just actively trying to take you down a peg for social reasons. And you have to either spend a bunch of time arguing about something that wasn't even the main idea you were trying to clarify or hash out, or not reply and look/feel like you're ignoring a critic.

(And because of inferential distance, the critique might look reasonable to an onlooker, whereas you've already had this conversation 10 times and know the various arguments and counterarguments, and don't feel like patiently explaining it again).

Abram's recent post on intellectual cultures explores the details here. There's a rough hierarchy of discussion that goes something like:

0. Open Verbal Combat
1. Face Culture.
2. Intellectual Debate
3. Mutual Curiosity
4. Exchanging Gears

Venues like LessWrong have a mix of people, some of whom are trying to do intellectual debate, some mutual curiosity, some exchanging gears (and occasional people doing pure verbal combat). Google docs lets you filter for people who are trying to do the kind of conversation you're looking for.

Collaborative Exobrain Technology

Google docs is legitimately great. Important features include inline commenting, realtime collaboration, suggesting text, version history, etc.

These individual features are good, and they add up to a really powerful "group exobrain tool." As of now Google Docs are simply much better than most other forum / blogpost technology for maximizing group-working-memory, and alternating between the "generation" step, "critique and discussion" step and the "distill" step of intellectual progress.

Some things that work better on google docs than many alternatives:

a. Peer Review.

The use case it sounded like you were most referring to. You can write up an initial paper exploring an idea. You invite smart people to comment on it. You might invite disparate groups of people at once, or you might take turns sending it to different groups of people who you think will do a good job of critiquing different aspects of the idea, or bring different perspectives.

The comments are attached to specific parts of the essay. In addition to immediately seeing the context of a given comment, I think there's something like "humans using physical space to augment their working memory" (similar to memory palaces) where having a "location" for each comment helps people keep track of everything going on.

You can also give people the ability to suggest text, and then have people comment on those suggestions. So brainstorming and critique seamlessly flow back into the rewrite process. Whereas on a blog, if people comment, then the author needs to set aside time and activation energy to turn all those comments into a second draft, and then wait for further comments on whether that second draft actually addressed the issues.

(Plus, there's something that feels "final" about posting something publicly, so doing a "second draft" feels a bit weird in the first place)

On google docs, the second draft happens sort of invisibly as you go along.

b. Planning.

Andrew Critch has been a huge proponent of using Google Docs to develop plans –if you have a course of action, be it a life plan, new project, or whatever, you can write it down.

Initially, writing it down forces you (at least somewhat) to notice what assumptions you've made, and make sure each step of the plan actually follows from the previous step.

Then, you show it to a couple people. At first it may not be very good so you just share it with one or two people as an overall sanity check. If you can find 4 people with different perspectives to vet the plan, and check assumptions you missed the first time, you can be a lot more confident that the plan makes sense.

The brainstorm/critique/distill/next-draft cycle applies to plans as well as idea-based-writing. But, in this case, the final product is not a paper to read – it's a course of action that people can commit to. If a plan calls for multiple people to commit to a particular role, then part of the drafting process is to make sure each role makes sense... and to find people who would be willing to do it.

Just like a google-docs paper ends with you suddenly realizing that you have a finished draft, with little activation energy required to commit the final comments, ideally a google-docs plan ends with you realizing you have a viable strategy and a bunch of buy-in from people who helped shape it and are ready to start turning it into actual reality.

So what do we make of this?

I still think you're right, that keeping all these comments hidden in drafts is wasting a lot of potential. I think there are potential solutions, but they require a fair bit of work, and accepting some of the constraints. People want the privacy and filtering while they're hashing their ideas out.

The main thing lacking with google docs is the ability to turn, not just the finished writeup, but all the corresponding comments, into a publicly accessible work, with a click of a button.

(The LessWrong team is looking into some options for making LessWrong posts more google-docs-like, so that people and orgs can use it for internal documents while significantly lowering the barrier-to-entry to sharing them eventually, but it's a big chunk of work and not something we can promise in the immediate future)