Probably because there's no 900RPM archetype of Autos anymore.
They might not have wanted to make it a 720 because then it'd be different to how people remembered it. Bringing it back at 900RPM is also a problem because then it's basically a 900RPM SMG but with better range, stability, and damage, so why would anyone use a 900RPM SMG anymore?
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Yeah, that is weird actually. I guess they just decided to pick and choose which weapons they were bringing back since Praedyth's Timepiece and Praetorian Foil also aren't being reprised despite being members of archetypes which exist in D2.