It's beause you probably only have a little bit of answers on it. I've noticed that too, sometimes my question is like 2 days old and I still can't choose most helpful because it only has like 4 Answers.
It's not useless, that tends to tick me off because I see how it was 20 minutes since posted, the question is viral, and the QA was like "OH MY THSI GUY AGREES WITH ME THIS IS HTUE JKBKEST ANSWR )1IKMA PICK MHO RIGHTE NOW" and i'm like seriously? It's been 20 minutes, and all this guy said is "I agree", yet whenever I try to select an MHO that early it gives me an error message. It SHOULD make you wait 24 hours.
I think it's either do to not enough answers or the admins conspiring against certain users. I've had it happen to me before, and it was six days later on a certain question of mine that the system finally let me pick most helpful. I'm thinking it's more likely the admins conspiring against certain users, as I've seen fairly recent questions with only very few opinions that had MH already picked. This question is far from dumb compared to some of the questions I've asked.