At this point, for most people, because it's one of the "banned" games that got hit with that initial wave of backlash from "payment processors are holding purchases they dislike hostage". Like, i absolutely could be wrong but the main reason i personally see it resurfacing in popularity is because it got round 2 of being a socially controversial game.

First round was stuff like the kicking puppies, pissing on people, and murdering protesters stuff. Second wave was the broad ban on any game that seemingly was a bit "too much" for censorship panels across the board (((mind you, yeah, some of steams games needed cleaned up, dont mind that aspect, hate how games like this got caught in the crossfire)...0 issue with "cousin Grape simulator 2.0" getting banned but a couple of actually decent games got drug down as well)).

Originally? Partly for "guilty pleasure" reasons, partly for "FAFO reasons", partly just because the scandal/taboo made it feel awesome even above and beyond the actual "FUBAR" stuff of the game.
Baseline? Because the entire point is personal freedom. What most people tend to ignore nowadays is that freedom comes with consequences as well. You absolutely will get assaulted/arrested/killed in this game for being a prick, doesnt stop you from being a prick and thats part of the fun...let the dicketry out in the digital realm so the rest of us can tolerate your otherwise insufferable self--kinda stuff.

Edit: it IS worth noting that the term "Postal/going postal" refers to a specific incident...nowadays often attributed to a conglomerate of incidents...of a postal worker absolutely losing their crap and turning homicidal/genocidal in the workplace, it is, at base, a social commentary game, but framed from a near (batman universe) "joker" style take on that level of burnout. so yeah its had a rather "checkered" past. Even with wikipedia's recently obvious biases, there IS a wikipedia post about the subject:
https://en.wikipedia.org/wiki/Going_postal