Before people go off on some long explanation on why it's hard to code and implement I already know this and not saying the devs should do it ....BUT... Because Mischief and Thornblade are now their own separate servers with the same rule-set...how cool would it of been to allow Beastlords and Zerkers start at level one with everyone else....with a catch. Beastlords able to start at level 1 on Mischief (No Zerkers) Zerkers able to start at level 1 on Thornblade (No Bst). This would of added to the randomness of the servers and given people a MORE personal reason to choose which server they started on. The devs could of made a bag set for each server and sold twice as much. I think it would of been cool at least.
In all likelihood, those classes turned on are probably bundled under checks for appropriate expansion unlocked (or equal to / greater than expansion number). It's probably not easy for them to just decouple the unlock from the expansion unlock. Not dissing DPG, just thinking that's the kind of code that was written under time and resource constraints in the moment.
The why it didn't happen has been discussed for years but concept was perfectly laid out for it to happen. Would of been cool and a big draw for players and profit for DPG.