Obviously we don't know ant might never know exactly why he got dropped but i think the best explanation is that 100T believes that Nitr0 can do as good or even better than steel as igl
Steel was by all means a good player but never really put up the umbers that Nitr0 did, by replacing steel they can add a sentinels main who frags more like dapr (obviously it wont be dapr).
As for replacing with b0i... i don't really understand, my 2 theories are that b0i is just incredibly underrated and has become much better since his time on andbox or that hes just acting as a replacement until a better opportunity arises