I think it became the center of the protest simply because it was the most popular Reddit iOS app and the developer actively spoke out against accusations from the Reddit CEO.
In terms of why it was so popular I can only tell you my own experience which started with bacon reader which like most Reddit apps at the time mostly mimicked the old reddit site. Then I switched to alien blue which used more of the iOS best practices such as larger buttons, swipes, etc. When alien blue turned into the official Reddit app, it became bloated with ads, suggested posts and other annoyances. At which point I switched to apollo which had the similar nice UI without the baggage of the official app.
But honestly I think I only got off the official app because of something annoying with collapsing threads that I have since forgotten.
In 5th grade they defined every kid that can speak another language as ESL (English as a second language) even if you spoke English perfectly. Then they put all of the ESL kids in a different class on the opposite side of the school. The result was that the school became de facto racially segregated with all Asian and Latino kids on one side and all white kids on the other. It’s not like it served a purpose anyway since none of the teachers could speak anything other than English.