Why does the apex of the human heart usually point to the left?
From a quick look at the paper @ChinmayKanchi links to (Palmer, 2004) it seems that:


  All living vertebrates possess a heart that is conspicuously asymmetrical and normally displaced toward the left (Fishman &amp; Chien, 1997). 


So the heart orientation seems to be evolutionary conserved in vertebrates (as are many fundamental traits), and no specific explanation is needed for humans. 

This is said with the reservation that human anatomy is not my subject field, and the refered paper also digs deeper into the molecular basis for the orientation/symmetry of organisms. For instance, it also says that:


  Second, the molecular pathway directing hearts leftward—the nodal cascade—varies considerably among vertebrates (homology of form does not require homology of development) and was possibly co-opted from a preexisting asymmetrical chordate organ system.


so the molecular mechanisms governing this seems to differ between species. This could indicate that there is a selective pressure molding species into the same heart orientation. This is pure speculation on my part though.

I also want to mention that evolutionary outcomes doesn't have to have a "reason" (i.e. a selective advantage). Much depends on chance events and evolution can only act on what is present at the moment (i.e. is restricted by earlier evolutionary history).