Shouldn't something happen to the people who ask a question that don't select a best answer or vote?
No... They do not have to vote or give BA. The answer could be wrong, they don't have enough points, etc. So they don't really have to.
If the Mods think it is the BA, they can give it to them.
Yes, let's fire them out of a cannon!
Seriously though, Ninja is correct. People don't have to give best answer. Maybe they don't think any of the answers is the best, or the probably don't know that they should give BA.