A lot of conferences have adopted a strategy of having talk proposal submissions open to the public. While it may be easy, and convenient for organizers to use something like pull requests on a Github repo, I believe it comes along with the side effect of turning off potential proposals.
I think people who would otherwise submit a proposal may be turned off due to public submissions for the following reasons.
Too much of a good thing
I recently submitted a proposal to a conference. I don't typically submit a proposal unless I feel like I have something worth sharing. In this case I had an idea for a talk that I thought was perfectly suited for the conference, and I was excited about it. Before submitting my proposal I ran my idea past a few colleagues to gain validation on the topic. The feedback was positive, so I formalized my proposal and got ready to submit it.
This particular conference had all the proposals that had been submitted publicly available. Before submitting my own proposal I found myself reading through the proposals of others. As I was going through the submitted proposals I came across one in particular that caught my attention. It was basically the same proposal that I was about to submit. In spite of the effort I'd put in to getting my proposal ready, there was a moment of hesitation. I thought "They're not going to choose both of our talks. They're too similar.".
In the end, I submitted my proposal anyway, but I knew the chances of having my talk chosen were lessened. The proposal process should garner excitement for the speaker candidate, not diminish it. Not to mention how many proposals were never submitted because of similar situations?
Sidenote: The other talk was chosen. Mine was not. At least I know I had a good idea.
I'm not cool enough
A large draw to attending a conference is to learn from some of the big names in your field. Especially at larger national conferences, it is almost expected that people at the forefront will be attending, if not speaking.
If an average physicist (if there is such a thing as an average physicist) is thinking about submitting a proposal to a conference, then sees that Stephen Hawking has already submitted a proposal, there's going to be immediate thoughts of inadequacy.
Keeping proposals private encourages submissions by preventing the natural comparison of yourself to other candidates.
They're all going to laugh at you
For people who are junior in their field, who deal with Impostor Syndrome, or otherwise are just thinking it's time to start speaking at conferences, it can be an intimidating prospect. Even for those who have done it many times it can still be nerve wracking.
Having proposals publicly available makes it easy for people to second guess themselves: "My proposal isn't good enough", or "I don't want people to know my proposal was rejected".
We should be encouraging anyone who has the slightest desire to speak. It can be a fun, and rewarding experience to share your expertise with others. Not everyone has the confidence, or natural talent, but We Are All Awesome.
In the end it's not any more effort to set up a form on Google Drive. Plus it will draw more proposals for those that are concerned about their proposal being made public.