Simple! Because people will still buy new "old school engines."
Look at the process to certify an entirely new engine to an airframe, it's pretty darn expensive. That cost has to be sent along to the customer of the mod and that's why many are still running around on thier JT-8Ds.
Although, there are some mods out there, but they're really pricey so most small airlines wouldn't go for them. But it's not too hard to find a 727 with Tays on it, usually a converted bizjet...
Dan