maybe you over estimated how many people actually want to play tiefliings?
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
