Umpires union I believe is the blocker there. I believe this was discussed in the last CBA to be added and the umpires union was the blocker. I think the umpires union argued it would make their job more difficult having to watch both bags. One to make sure the fielder has their foot on the bag and the other to watch for the runner.
I think just making the first base bag bigger on the runners side would make this far easier than adding two bags and trying to get the umpires union to agree with this.