TLDR mocha (not entirely true, i read like half) but to form my own opinion,
I believe that twinking on a server is like growing crops on a farm, with your crops being the players and the earth being the server (i know its corny).
If you look at my server, for example, i have a few twinks in different brackets so me sitting on the bank steps with my glaived rogue, i get comments from people saying like "Wow nice weps, how is 70 twinking?" I mean i don't play it much, but ill generally tell the person a response not too different from "The queue times are great, its a good community as a whole, and a lot of fun!"
I feel like me doing this sparks interest not only on my server, but on twinking as a whole. This was just an example, as i have done this with other brackets as well.
If everyone is on a seperate server, there will be a higher population to pool players from, adding activity to the bracket (even more since "battlegroups" are not what they used to be.
If everyone was on the same server, twinking would be well known on that server sure, but on a server without any twinks, noone will ever hear about it in their WoW career. (even more so since twinks are in different bgs so they won't even get the exposure that brought so many of us into this community)
Sorry for my long post, hopefully someone got somthing out of it