Maybe a little history can help explain the current state of D2 speedrunning.. I recall Px and possibly also P8 categories were originally part of the speedrun.com leaderboards back in 2013-2015? The D2 speedrunning community was much smaller back then and P1 was the most "familiar" category to the overall D2 community, most likely due to Bnet I'd guess. The mods removed the largely empty Px/P8 leaderboards due to inactivity/lack of interest from the community. When D2 speedrunning started to take off on twitch, the hardcore runners wanted their own leaderboards since there originally was no differentiation between SC and HC in the Normal and Hell Any% categories. There was enough demand from the community and thus the mods added SC/HC categories. For a few years there were only the 28 P1 categories: 7 characters x 2 difficulties x 2 modes.
I think it was 2 years ago in 2018 when Px/P8 categories were added back into the leaderboards. For a while P1 remained the most competitive category with the pinnacle, in my opinion, being the race to the first sub 1 hour P1 Any% Sorceress. The "Jail Strat" that made the sub 1 hour time possible ultimately lead to the decision to ban "-ns" and to use an "ingame timer" that paused the timer during the notorious S&E hang time. This decision was just one of many topics discussed during the May 2020 rule set discussion among D2 speedrunners. Some parts of the current rule set may seem weird or arbitrary, but it's a culmination of compromises made between current runners, viewer experience, mod availibility/VoD verification process, the submitted times of past runners, etc.