Why aren't 998 and 999's more popular?

Lockup converters have never been favored for high performance applications and as such converter selection for performance usage is limited. If you are building a mild street car then maybe it's a good choice. If you are building for performance and don't care about fuel mileage than it's not a good choice. In addition NHRA does not allow lockup converters in many classes of race cars so there is another reason the aftermarket doesn't support it.