Because they have totally different bonuses. How would you stack bonus with threat generation? It's easier to just stack +3% more dmg with melee for example. In my opinion it's fine just like it is now. You have plenty of rings to choose from right now. Bel, ToMM, mod 19
The new rings just recycle the Underdark ring bonuses and so inherit their limitations as well. However only the melee and ranged perks only stack with TOMM rings and then you may end up with either way too much crit or accuracy.
"We have always been at war with Dread Vault" ~ Little Brother
Comments
It's easier to just stack +3% more dmg with melee for example. In my opinion it's fine just like it is now. You have plenty of rings to choose from right now. Bel, ToMM, mod 19