but postcolonial theory is decades old, why are they all acting like it's something that only sprung up recently?

because now people are applying it in ways that pose a very immediate and significant threat to western interests in the middle east, and discourse about how palestinians are somehow the colonizers didn't stick, so the best move for maintaining western hegemony is to denounce it. up until now, ignoring it or paying small inconsequential lip service towards it was the better move (because jumping the gun on that would end up being received a lot like republican fearmongering about CRT), but that only remains the case as long as the material impact of it can be contained.