I think it's mostly a matter of costs vs. benefits. 4D coasters are immensely huge, have trains the size of buses with terribly many moving parts to maintain, require a complex support system, and hence carry a pretty hefty price tag both upon purchase and for upkeep. And they take up a lot of land and require a favourable relationship with whatever height or noise regulations that are in play. Sure, they are impressive and enthusiasts love them, but maybe they do that a little too much - practically speaking, they are the hardest-core thrill coasters you can buy, meaning that only a somewhat narrow segment of your target market would even want to ride them. And overall there is only a tiny handful of parks that could want to build one of them in the first place. Worldwide, that is. They're out of the price/scope/regulations range for all but a few players.
I think most of those parks would consider their options, weigh a bit back and forth, and find out there are better ways to spend their money. They could get an even bigger coaster for the same price tag, with a wider market appeal. Or maybe a coaster that carries the same white-knuckle factor for the thrillseekers (say, a Dive Machine or a launched multilooper - most park guests would consider them just as impressive) but costs way less. Heck, I wouldn't be surprised if it turned out that the Free Spin coasters draw almost as many guests to the Six Flags parks as a 4D like X2 would, and adjusted for costs I don't even think there's a contest.
There's also the argument that 4D coasters are notoriously inflexible when it comes to layouts (you're practically forced to start with a vertical drop and a raven turn, and then do it somewhat compact from there, much like a Dive Machine) so that even for the great price, you're not getting something unique - the coaster would probably resemble X2, Eejanaika and Dinoconda - but I'm not sure how good of an argument that is. Parks build pretty samey Dive Machines all the time, for instance.
Maybe the 4D coasters will stand as the high water mark of crazy coasters this century. Very big, very thrilling, very expensive, and all around not very practical when considering the cost. The industry has seen a shift recently into smaller coasters that deliver white-knuckle thrills on a tiny footprint at a fraction of the cost of the huge multi-loopers that dominated the industry around the turn of the millennium. Maybe time and technology just moved away from the 4D coasters.
