I think one of the original drivers was the ability to quickly tweak parameters, after Linux rejected what I think was userspace adjustment of window sizing to

I think one of the original drivers was the ability to quickly tweak parameters, after Linux rejected what I think was userspace adjustment of window sizing to be more aggressive than the default. The Linux maintainers didn't want to be responsible for congestion collapse, but UDP lets you spray packets from userspace, so Google went with that.