I have been itching to ask this question. A while back, all sites adjusted to +10 for questions and answers, except area 51. Why? Is it why we start with 51 rep?
Area 51 runs on a very old Stack Exchange codebase, so changes on regular Q&A sites aren't always ported over.
One of the reasons could be (but I'm just speculating) that the change on regular Q&A sites was to make sure questions were awarded just as much as answers:
We’re changing the reputation earned from getting a question upvote to ten points, making it equal to the reputation earned from an upvote to an answer.
There are no answers on the main Area 51 site, so there's no need to increase the reputation gained from an upvote to the same level.
You have to compare it to other reputation-gaining events; is an upvote to a question really worth twice as much as someone following/committing to your proposal, or successfully referring someone to a proposal?