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.
Good point. P2Pool for Bitcoin still use PPS, while P2Pool for Monero these days use PPLNS.
I inspected it and it seems to be block mined with P2Pool software. I can't access website mentioned on it's coinbase tag, but i wonder whether they only use abandoned P2Pool software or did some modification.