The Gemara said (taken from the Point by Point):
(a) The Persian bathrooms were considered covered - even when there was Tzo'ah in them, and it was permitted to recite the Shema in front of them.
(b) That is because the hole was situated at the end of a slope, so that the Tzo'ah would roll away and end up out of sight and of smell.
What could have been the reason that the Jews did not adopt this simple chochmat goyim to build their bathrooms the same way??
Harold Jitschak Bueno de Mesquita, Jerusalem
1. It may be that some Jews did adopt this design. They were sufficiently widespread to justify the Shulchan Aruch's (OC 83:4) mentioning of what the Halachah is for these bathrooms.
2. However, it appears that many Yisraelim did not use this model. We see this from Talmidei Rabeinu Yonah (page 17b of the pages of the Rif, paragraph beginning Batei Kisa'ei), who cites Rabeinu Hai Ga'on who says that the Persian bathrooms were on a slope so that the fluids ran away immediately, while in contrast our bathrooms are not on a slope. I would suggest that the Persian bathrooms required somewhat deep digging in the ground and therefore may often have been difficult to engineer in practice.
Kol Tuv,
D. Bloom