There are two issues: Early cars had oil ring problems which caused hot oil to pass onto the precats, which in turn broke up and got sucked back in and knackered the rings more which cause more oil to pass etc.
The other problem is that the precats were made of very cheap ceramic material, and tended to break up very easily on their own. They then got sucked back in, etc.
FL cars don't have the ring issue, but they'll always have the precat one. 2ZZ option would've been useless, at the time the MR2 was already more expensive than the MX5 and was much less practical, no-one bought them at the price they were up at and a 2ZZ would've been at least £5K more. No-one would've bought that either! Well, you and I would, but Toyota would've lost a fortune on it. If the TTET kit had been out with the FL then that would've helped, but it didn't launch until just before they stopped selling the car, and even then only in a handful of places.