The thing to remember is that the accepted answer can be changed, and if a user is engaged and familiar enough with the system to accept one in the first place, they'll probably be likely to change it if something compels them to do so.
Fifteen minutes (the current barrier) is sufficient on sites where a question is likely to receive several answers in the first few minutes. It's designed to not put off other potentially better answers by the presence of the check mark.
On smaller sites, it's possible that a few answers may come in within that time, but that's not quite always the case. If the limit was put off to an hour, it's very likely that the person wouldn't remember to accept at all, as presumably they're off to do something with the information they received in the answer. Hence, it's a bit of a balance.
We want the value to be consistent across the network otherwise we run into the pains of documenting where it isn't the same, which is a major headache. Even though it's possible, I don't think we'd want to start making sites unique in this regard, and the 15 minute window seems to be working out well in most situations.
If a clear pattern emerges that questions get one, and only one answer and it's clear that the accepted answer is the culprit, then we really should be looking at UI changes so that folks are more strongly encouraged to answer anyway if they have more to add.
I won't argue that this doesn't happen from time to time, but not really in enough of a frequency to make us think about bumping the minimum network wide.