Back in the day of EoE and maybe even RWF 1 I'm pretty sure dwarf could get projectile protection, but it could also pretty much infinitely stack every enchant and it made for some pretty lame gameplay (especially in late game). In the current state of the game, dwarf is already pretty powerful and if you can play your cards right you can usually win a 1v2 or even 1v3 as a high level dwarf in late game. Adding projectile protection (assuming you want it to stack similarly to protection) would mean after about 20 levels a dwarf could have Projectile Prot V... as well as prot and blast prot.
I think dwarf is in a pretty good place at the moment. Adding projectile protection would severely weaken any attempt at countering it and make it extremely powerful, especially in smaller games. Dwarf is a pretty situational kit and its only real counters are bow kits, which I think is pretty fair.