Because it's possible to flake...... so you have to trust someone with the job they did coating it, kinda of like trusting a painter and his paint not flaking, or a powder coater.... etc. An Iron sleeve is a big hunk of metal, not gonna flake, just gotta hope the installer installled it correctlyI wasnt sure about nikasil either, but it is a carbite based substance that is actually harder than an iron sleeve. I dont see any reason why it would wear through unless you didnt break the bike in properly and revved it cold. Theres nothing wrong with sleeves though.