Because long questions make the site look messy and usually don't catch the interest of the readers according to some answers I've read before.
I don't mind the character limit, but I wish they would implement a details box.
Why are the questions limited to 250 characters?
You can ask your Q, then place the first answer to further explain your question. So there ya go, an extra place to ad to your Q.
From the users that I've talked to on here, most won't even read a long drawn out answer, let alone a long question.