I also believe the sheer size of the pre-fabs could be an issue too. Three of the four prefabs are among the top three(!) tallest wooden coasters currently in operation, and the last is the seventh biggest pure wooden coaster built after 2000 - again, of which the top 3 are the other prefabs. In the relevant time period, only Thundercoaster, Wodan and Coaster Express have matched the four prefabs in size. The market for so big coasters made out of wood can't have been that big to begin with, even before RMC began eating into it too. When the prefabs were introduced, most parks who could afford/want a huge wooden coaster had already got one. The rest of the woodie market is dominated by rides which are a lot smaller, and for small coasters the pre-fab production method is less competitive.
Speaking of competition, gavin hit the nail on the head too. "It's a coaster type". And since the turn of the millennium, when prefabs started hitting the market, we've had a bit of a revolution on the coaster type front. Seriously, look it up, and see how many of the ride types we now take for granted didn't exist before 1995. Parks used to have a rather small catalogue of coasters to choose between, for big thrill rides you could basically boil it down to "large steel coaster that loops", "large steel coaster that doesn't loop", or "large wooden coaster, which cannot loop". This is exaggerating slightly, but it's still astounding how much the selection of coasters has ballooned in recent years (another example: try playing RCT2, which included pretty much every coaster type known to mankind when it was made, and count how many currently-popular rides are missing). The pre-fabs are competing in a market which is a lot more diverse than it used to be only a decade earlier. It's no longer just wood vs. steel, but wood vs. hyper vs. looper vs. dive machine vs. invert vs.... you get the point. For huge thrill rides, parks simply have so many options nowadays that woodies kinda disappear into the mix. And traditional woodies might be perceived as harder to market, seeing as they can't perform inversions or feature fancy trains, or for that matter launch (getting the ride up to speed with a launch, as opposed to a huge lift hill, saves tons of money in bureaucracy and construction costs). The big woodies that get built nowadays, are those modified to do all those things woodies normally can't. Prefabs are simply a little too old-fashioned in today's busy market, and the market was nearly saturated already when the big boom began.
Here's hoping for China, though.
