There were several credible competitors: WebOS, Symbian, Maemo/MeeGo, Palm OS 6, and of course Windows Mobile. But they required cooperating with a single vendor and/or a lot of work to adapt.

Android won because it was available "right now" and easy to hack. Vendors could get a BSP (Board Support Package) from a chip manufacturer, slap Android userspace on top of it, and ship a phone within half a year. It was a glorious mess for a while.

Google then slowly tightened the reins and made the ecosystem more ordered.