I think a big factor is how you are raised. I'm just using myself as an example, but while a lot was expected of me based on the aptitude that I showed, there was no one that ever demanded excellence. A couple of teachers told my grandmother that I could have skipped a grade if I put my mind to things, but as long as I knew I could do just enough to pass, why try any harder?
I feel like if there were consequences or, better yet, rewards for making A's, then that would have been ingrained in me. As it stands, I'm now an adult that, you guessed it, doesn't aim to please anyone and is content with "just enough". I don't say this as an excuse, but more as an underlying reason.