Because the topography of the place makes anything other than busses pretty much impossible. The name "Wuppertal" translates literally to "Wupper Valley", which is pretty narrow. I find it awesome that the city was once very wealthy, so they decided to just go for it and build something cutting-edge for the time. Nowadays, the initial investment would no a big no-no.

I've been on it. It was cool as fuck.

(Was visiting at some point around Gamescom nearly a decade ago.)