It's mostly Reisz's character design holding up over the years. Also she had a cult following before rule34. A really, really cult following given the SNES was dying. Several artists had childhoods shaped by the decade of Reisz porn being the rarest of the rare. Finding all they wanted of her was destined to die an impossible dream.
And then, seemingly in defiance of the universe, the stars aligned. Even before nostalgia kicked in, early polygons lost their luster and the world glanced back upon sprites with wonder. The age of moe took hold, plagued by cheap exploitation... yet figures of bygone eras who matched its appeals only shined brighter without sacrificing dignity. Rule34 exploded across the internet followed by the growing allure of retro video games. Amidst the chaos a voice shouted "Reisz!" and was echoed by hundreds. Those children of the '90s stood their ground against the all-consuming void of obscurity that had fed on their hopes all their lives, and on that day they slew an immortal beast. With every work dedicated to their Valkyrie, the fans live the dream and celebrate victory over the impossible. And god bless those crazy bastards, may their banquet never end.
Basically, Reisz keeps getting porn because she's pretty cute.