A possible reason for this is explained in the FAQ on Meta Stack Exchange
There are some differences, though, that exist in order to ensure everyone gets a fair shot at answering
Users are less likely to write an answer to questions which already have an accepted answer. You might be satisfied (and rightfully so!) with the answer you wrote, but we're ...
The Help Centre has:
You can also accept your own answer, but you must wait 48 hours to do so. After all, someone else may come along with an even better solution to your problem!
That link leads to the blog entry Jeff Atwood wrote on the subject.
Now, there are some special rules around owner-accepted answers, to
Wait 48 hours. You must ...