Limited support towards some ASIC also the reason it's dead. Meanwhile P2Pool on Monero is alive and well with about 5% global hashrate.
That was probably a tiny little reason, i think it was compatible with most gears out there, what really killed it was PPS payment method, it nearly killed all non-pps pools, p2pool "died" a lot faster because on top of it not being PPS, it was not for the average joe.
However, p2ool lagecy still lives if you dig enough, if you look at this block
https://mempool.space/block/00000000000000000001822dc3db70b75d281687f8baa10d1818d0703f49fec0There is a clear evidence that this is p2ool based block, look at the number of output addresses, and the coinbase tag, it refers to a p2ool node based in Russia.