When you know you want to use it, then you know that you need to change settings (even if you don't know which ones).
That's the problem. Most people don't know they want to use it until they've wasted a lot of time trying to figure out why they are seeing garbage, or why people aren't responding to them (because the other people are seeing garbage as well).
My gut feeling says that there would be very few people wanting to switch *away* from UTF-8 were it the default. Although admittedly, those few people would be very vocal about it..
If it's troublesome explaining how to set it up correctly in your channel(s), then use a script to explain it through /msg, /notice, or even /dcc send'ing a file. You can have an on join message telling people to type !unicode or whatever the first time they join in order to get set up correctly. It's automated that way.
Onjoin messages are a significant annoyance to most people. It would be much more helpful if UTF-8 were the default, then I could just tell people to install mIRC and everything would work out of the box. I'm a firm believer in the zeroconf philosophy. Type in any language and boom, it works.