The reason I heard is that Red Reader is totally free and open source, unlike most others. It also has accessability features that the official app lacks.
Quantum Badger, the developer still seems quite pessimistic about things though and he hasn't had any assurances about how long the exemption will last for, from what I understand.
He's talking about adapting the code to make it more Fediverse compatible, so hopefully Lemmy compatible in the not too distant future. This has to be the way long term now that Reddit is heading towards IPO.
I've been a Red Reader user for years, from the time when Reddit is Fun went from open to closed source and it's a great app. I think I'm ready to quit Reddit now and start exploring alternatives and I'm sure in only a few months there will be new apps that work with the Fediverse which will offer a Reddit-like experience.
Once the critical mass of users is achieved it should hopefully scale fine, with a diverse range of instances absorbing the load influx.