It's because too many things are defined in terms of UTC; if you start saying, "Okay, let's start using TAI from now on.", now you have the complexity of trying

It's because too many things are defined in terms of UTC; if you start saying, "Okay, let's start using TAI from now on.", now you have the complexity of trying to remember whether you're supposed to use TAI or UTC for this particular protocol and/or data format, so in practice people will stick with UTC there. On the other hand, removing leap seconds from UTC is trivial - they're inserted manually anyway, so you just stop doing it.