Three main reasons, as I see it:

1) Most Antminers (including the S4, S5, and S7, and possibly the S3) have problems with p2pool, and will lose hashrate and become unstable unless you use a special firmware. These problems were fixed with the S9, but during the earlier generations, p2pool lost a lot of hashrate.

2) As block sizes increased, the CPU load on p2pool nodes has increased as well. In order to get reasonable efficiency with p2pool, you now need a fairly fast CPU, like a 3+ GHz Sandy Bridge or faster. Network bandwidth and latency requirements have also increased.

3) There's no Chinese installation manual for p2pool.