4 factors

We know that the better approach to learning a new piece (especially at the beginner's level) is not to play the piece from the beginning until the end or play all parts together right away.

Yet I see some of my students do it all the same. I know these are efficient practicing techniques and I could prove from many examples in my or other people's practice that they do work.

So the question is why we still practice the wrong way when we know there is a better (not necessarily the best) way?

I think the answer depends on several factors:

1. Worldview 
2.Trust
3. Care
4. Resistance

The person whom we want to change has a different worldview than we. In addition to that all interactions are based on trust. If the person trusts us, does he/she cares enough to listen to what we have to offer and take action? And finally, even if this person trusts us and wants to take action on what we have said, he/she still has to confront his/her own internal resistance.

We can't really change a person's mind to start practicing the way we believe is a better way. The change has to come from within.