Because it automatically closes after you select MH or their is no new opinions on your question after a certain amount of time. Where have you been, closing of questions have been this way for a while now.
Mine generally close after 4 to 5 days. I think it also depends on if the question still gets new opinions. Once that stops, I think it soon gets closed. I must say it doesn't really bother me, since after 2 or 3 days without new opinions generally nothing more will come :o
Cause they want to ask more questions. I don't close them too soon, I wait a little bit. I want more people to give opinions. Oh yeah, I'm afraid people will be so annoyed if I keep asking too many questions everyday.