I guess the only reason is because there are so many. Let's say we lost most of those options, and those were the only two contenders line which would you choose. That's all
I use Jerboa regularly, it's my jam. But I don't know why we'd need to limit ourselves to two anyway, even hypothetically. The myriad of apps with various features is Lemmy's and the Fediverse's strength.
Let me add to this by saying if there's something in Jerboa you don't like, you have the opportunity to change it by contributing.
Jerboa didn't have Mark As Read On Scroll so I added it.
That's why Jerboa is the best. Also, Boost has ads 🤮