Stability.

Computers were far more crashy in those days, but OS/2 crashed far less often than Windows or even DOS did. And sometimes when a program crashed on OS/2, it only killed itself; it didn't take down the whole machine, so you had a chance to save your work in other programs before rebooting.

It also either was, or felt like it was, very very fast. Windows felt like a laggy VNC connection. GEM and the rest weren't much better speed-wise than GEOS on a Commodore 64.