darjr
I crit!
Honestly I don't think either of those fit. I don't have a better option to offer as to why they did this, however.Paizo was not a problem for them. Or else they'd have tried this during 4e in order to shut Paizo down. What WotC just did was an attempt to control 5e(minor goal) and much more importantly, gain full control over the VTT arena(major goal). The older editions were just collateral damage and not a target of WotC.
I do think that within wotc there were some that were earnest about the "hate" part and some that thought clawing back revenue from 3pp was a good idea too.
But the essential reason? I don't think we really know yet.