Because the people who got the Harrier working couldn't get it working and developed the F-35 instead, because that was the solution to the problem.
Excuse me if I am speaking out of turn.
You mean that Boeing, aka ex-MDD didn't know about hot air ingestion and choose the wrong path , while all along they had the AV8B in their portfolio?
LID devices ,when the gunpods wheren't installed, was something they came up with, no?
Also the hot-air gasses on the X32 are a good deal further aft compared with the Harrier, hard to imagine that with some tweaking they couldn't get their system working adequately.