But, there's the rub.  See, 5e doesn't actually use 1-2-1 counting.  So, you can move diagonally 6 squares on a grid.  But, for some bizarre reason, spell effects still use the 1-2-1 counting.  Sort of.  It's actually the worst of all worlds.
In any case, it's easy to explain why fireballs are square on a grid - ease of play and a gridded map is not, and never has been, a perfect representation, but, rather an abstraction.  So, we have square fireballs, because it's a heck of a lot more convenient to play.
edit to add -
Hang on.  Neat pixelated cubes?  Sorry, that's 3e, not 4e.