Killing 3rd party apps lets them funnel traffic to the official app instead, or the official website. Not sure if you've compared 'old' reddit to new (on desktop), or the app to 3rd party ones, but there's a lot of gamification, ads, bad patterns they're pushing with what they have. I suspect it's largely to monetize on the user more significantly via tracking data and such.

I assume they'll kill off old reddit shortly here, once they implement the API changes.