Because at the time, the Stadbahn was still alive whereas the PCCs and Interurbans had all died. Darwinism happened: survival of the fittest. Also such units worked out well for San Francisco's Muni, which (along with Pittsburgh's and Columbus's systems) the last of the prewar trolley systems. Lines that couldn't fit this were either dismantled and replaced with a typical metrorail (BART over the Sacramento Northern, the Skokie Swift) or upgraded into regional/commuter rail (Caltrain, Metra's IC route) or repurposed for freight (what's now the Alameda Corridor). When given declining ridership, declining interest, and declining tolerance by Congress there's only so much you can do. In general, this meant most service was discontinued. Stadbahn-styled units offered greater capacity given existing infrastructure, and required no new training or capital expenditures. New systems (San Jose, Sacramento, LA) went for it due to the low floor boarding options.
It is worth noting of those three original systems I mentioned, SF's Muni suspended all rail service months ago and expects it to be suspended for at least another year or so due to deferred maintence. But given how BART has always wanted to destroy Muni, the problems Muni has with their Central Subway project, and how BART is pushing hard for a second Transbay Tube right now, it is plausible that Muni will succumb. SF's city government is more than happy to toss it all away, today, because as far as they are concerned all cars will be self driving by 2050 and the larger Metropolitan Transportation Commission (regional transit authority for the Bay Area) wants to punish high ridership systems if workers do not remote work (which in their view, will reduce ridership to an acceptable level consummate to the amount of available capacity).
