I think the "issue" is probably mostly that races have very specific stat bonuses and classes are stat-dependent. So basically, why play a Tiefling unless you want to play a Warlock? Sure you could also go Wizard, or Fighter if you take the Zariel variant, but then you can just as well play a human and get 3 additional stat bonuses. It's either that, our build your character sub-optimally
Except that almost no classes can make much use out of those extra 3 stat bonuses, and humans get absolutely nothing else of value, so it is at least a little strange that human is the most picked.
