Why is there a question limit on Area51?
Because I was asking too many questions.
Once a given proposal has a hundred questions, few people are going to go through all of them and vote for them. If only a few people are allowed to provide all the example questions, then the site has little "community" input - it's really being defined by a few people.
By only allowing each person to contribute 10 questions, a site needs at least 10 different people with different viewpoints to get a hundred examples.
In short, it prevents a few people from monopolizing the conversation.
Forthermore, the questions are meant to be the defining questions of the site. It's easy to generate a million questions that would definitely be on the site, or definitely be off topic. It's much harder to generate questions that intentionally explore the gray area and force people to really decide what the site is about. By limiting the number of questions people can ask, it is hoped they will choose their questions more wisely*.
This problem is also helped by being able to vote on deleting poor example questions.
*Of course the most efficient method is using a binary search, and each user can, by using their 10 questions effectively, define the site accurately to the 1024th degree.