1. Third party app users represent significant enough lost revenue to make it worth making the API prohibitively expensive for third party apps to run, forcing those users back to the official app.
2. Third party apps users are few enough that Reddit can afford to not compromise and lose those users just to kill off third party apps.
Are they expecting the third party app users will come crawling back to the official app? As far as I’m concerned the official app is borderline unusable compared to Apollo, for example, and I’d expect most users who have seen the light to find the official app completely intolerable.
Perhaps the presence of unofficial, basically un-monetized third party apps was flagged as a risk for their IPO by some investor types. Still, they could have found a better route to monetize those users than just killing off the apps altogether.