This really isn't a question of why is which one better, but it is a question of which is more appropriate for a given application.
If you are making more than 100hp, then straight (involute tooth) gears are probably worth while as they offer less resistance and torsional load to their repective shafts.
However, straight (involute tooth) gears generally hve more backlash, wear faster, and are much louder than helical tooth gears.
If you don't drag race, make 100hp or more, or thing it's fun to make gears yourself, you don't need straight cut gears. Go over to PS and they will tell you that all bikes should have them and if you don't immediately spend $1k on them you are a loser......................, but then again that is Planet Sand and it's full of idiots with credit cards (which includes me). :geek: