Winter Wonderland has grown tremendously over the years, and a big part of that has been the increase in the theme park / attraction side of things. There's a reason why Olympia Looping, one of, if not, the, most famous travelling rides in the world, comes every year. And it's the only place outside of the official Oktoberfest setting where it uses 7 car trains. That's not just because of "a few enthusiasts".
WW works well because it has everything - the rides, the beer tents, the music, the market stalls and everything else. Take away any one of those things and it won't be as popular as it is.
Also, 2.5m people every year aren't flocking all the way to Hyde Park to drink in a tent.