Apollo was big in the headlines because its developer was the most vocal about the API changes.
As for why people used third-party apps, it’s mostly a preference thing. Something to note is that reddit didn’t always have an official app. Everyone using reddit before 2016 had to use a third-party app if they wanted to use reddit on their phone. A lot of the apps we watched get shut down, especially the ones on android (RiF, Sync, BaconReader, etc) have been around for a long time, and had loyal user bases.
Apollo was younger than the official app, but it was likely favored by those who had used Alien Blue (a very popular third-party app for iOS that was bought by Reddit and turned into the official app)