The main reason is that libtorrent, which is the literal backbone of most torrenting clients, has implemented supported for I2P only recently in its latest v2.x branch.... It takes time for libtorrent to iron out bugs and stablize and it takes more for clients to upgrade their embedded libtorrent to v2.x.
libtorrent 2 also has some issues. On unRAID for instance it causes crashes so I am forced to use v1 builds. And on other systems it has high memory usage so it's not exactly ready for prime time.
I hate to put it this way because libtorrent is a wonderful piece of open-source software maintained by volunteers but as is typical with its history, releases are going to be bumpy.
Yeah my comment wasn't a knock at the software or devs. I just think libtorrent v2 is not quite ready for widespread use yet. Since OP is talking people migrating to I2P then it needs to be more stable before that can happen. A few years from now I'm sure it will be a great option.