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.
i've messed around with i2p, it requires much more technical knowledge than just torrenting and there's a bit of a learning curve to navigating and configuring it. plus it honestly hasn't changed much in years, and i'm not sure how much i trust the developers working on it. i do agree that it's better than tor though
From what I have read on the upcoming implementation on i2p in qbit, I do not forsee it being adopted by the core torrent user base. The main issue I have with it is that while you can download from clearnet peers, you are only able to seed to other i2p peers. This completely eliminates any adoption by anyone using private trackers. Its not like the guys in brazil are going to jump through an extra hoop to hide their ip since they never needed to in the first place, so they can be ruled out for adoption as well. What I really fear is that its going to create a completely unnecessary schism in the userbase with a sort of 'leechnet' walled garden of i2p users which would hurt the greater availability of seeders. I also haven't gotten a straight answer from anyone how ports are going to work in i2p, since normally its imperative to have a forwarded port in order to be a full participant. Unless i2p users can seed to clearnet users without issue, I am going to be worried about the impact on torrent health as a whole.
I think non technical people find the concept of a network within a network confusing. I think its more of i2p itself.
I do think it is the future of filesharing and if I was to create a torrent I'd use i2p. Hopefully with more clients implementing support it sees further adoption.
It supported torrent for ages, but it has only one tracker that doesn't really have very much content. Now qBitTorrent or something else supports it out of the box.
I2P is very slow, slower than Tor. Maybe after more people join, it will be faster. Last time I tried it was painfully slow to even load most eepsites.
Because I'm not knowledgeable enough to make it work. I've managed to install it and figured out that I need to run the restartable version. Lots of text with screenshots or a video(s) would help. I'm not a digital native. This is one of many retirement projects. Listening to more music than I can afford to buy greatly improves the quality of my life.
I've used it to test, I don't do it because I can torrent fine on clearnet and I feel like doing it is needlessly congesting the network, consuming traffic that I don't really need.
I would also say that this is because anonimity is not needed by all countries. I know that in Spain torrenting is not a big issue and ISPs don't care that much (even though distributed Copyrighted content is still not allowed). They usually go after people profiting from distributing copyrighted company rather than people downloading or distributing for free.
Disclaimer: I think some of my info is a bit outdated so if anyone has more recent info about Spain's situation please tell me.