Why ? We have No Baseline believeing that they would do better with toss or zerg.
But why ? We have No Proof that terran needs to try Harder or is Harder in any Kind of way.
I too think thats why they are the biggest Population. Partially.
The Problem with the biggest Population is that it attracts obviously a wider range of Players. And because terran attracts a wider range of Players they also attract the Players that Just want an easy access and Play the game. And with terran flooding the Casual leagues (and leading Like every other League too) its Safe to say its because they are Bad and Not because the Race is inherently hard, because as Said there is no cutoff.
150 apm would be high for those Players in Question. We Talk about Bronze Silver and maybe gold. If we Look at sc2replaystats those Guys in Bronze and silver have an apm of 60 to 70. In gold sub 90. No one can Tell me their apm is so Low because of Balance. No one can Tell me they are flooding those leagues because of Balance.