The problem was that if you used a third-party client, Reddit would have to coordinate with them to launch whatever new stupid cryptocurrency scam they wanted to push that week. On a web browser they can just push new code into it[0], and their first-party mobile clients can be updated ahead-of-time with support for the feature. But third-party clients would have to spend their own development time adding stupid "click here to get your Snoovatar[1]" links. They could slow-walk that, or just not implement that, and Reddit would have to spend time and money kicking users off that third-party app.
This, incidentally, is why every other major social media platform bans third-party clients. Third-party clients are user agents, not platform agents.
[0] Which, incidentally, makes web browsers not user agents
[1] An NFT scam Reddit tried to pull