I think it’s a way of attempting to assure quality, but I’m not sure. They also may not want to inundate people with so many questions that no one answers many of them.
I think it’s more about spam control than anything else. Since the questions don’t go up for approval first, someone could potentially spam the site very easily if there wasn’t the question limit.
[mod says] The answers above are correct. It’s to encourage people to ask high quality questions, and to protect from spam attacks. Welcome to Fluther! :)