Because they need to be A) squishy, and B) melee.
If they didn't have disables, they'd need a ton of damage avoidance/resistance, and then they aren't squishy. And with the combat utility removed, then they're just stealth tanks that deal a lot of damage.
In PvP, utility generally falls into one of two camps: offensive CC, or healing/anti-CC. If they wanted, they could take out the knockdown on opener and remove Dirty Kick and Flash Bang. But a lot of people who play a stealther don't want to heal, or buff allies as part of their full-time duties.