Back in 2005, a bug report was filed by Kjetil Kjernsmo, then running KDE 3.3.2 on Debian Stable. He wanted the ability to have each connected screen show a different virtual desktop independently, rather than having all displays switch as one unit.

Over the years, over 15 duplicate reports piled onto the original as more people ran into the same wall. And that's not a surprise, because multi-monitor setups have become increasingly common.

The technical reason why this issue stayed open this long comes down to X11. Implementing it there would have required violating the EWMH specification, which has no concept of multiple virtual desktops being active at the same time.