DSRC and C-V2X are both in their infancy. Allocations were made in the 90s when ITS infrastructure was just taking off, but autonomous vehicles took longer than expected. Note that Pai parrots the talking point about DSRC's low deployment, but never cites numbers about how C-V2X is even less deployed. It's bald-faced, but journalists don't seem to be calling it out.
To put it very simply: DSRC is a peer-to-peer technology, and inherently allows more individual privacy, since you're only communicating with vehicles around you. It's using the same silicon as wifi, meaning there are lots of potential vendors for it.
C-V2X is being pushed by a certain large company that makes a lot of cellular silicon, because it uses their cellular silicon. They've been throwing large, large sums of money at trial projects and lobbying to kill DSRC. It appears they've now succeeded in that.
Enjoy your single-vendor panopticon future, everyone.