Originally Posted by Stacey
That, and you needed the long horizontal dropout for chain adjustment on single speed bikes.
Yup. SS became rare as everyone became able to afford a reliable, good derailer, so makers stopped caring about making their frames SS-friendly. They rather made the dropouts more derailer-friendly and foolproof (loose QR, anyone?). Given how SS/fixed is spreading like wildfire, horizontal may come back from the grave. I hope it will.