Because that's the mainstream, that's what's easy to procure, hire for, negotiate with vendors, that's the default, the preferred, the future proof, the supported.

And the tech doesn't really matter. I hate React with a passion, because Angular is so much more sane - in my experience. But it's fine. It's mature, it can be made to perform completely well.

The tech doesn't really matter. The people doesn't matter either. Even the costs doesn't matter as much as people think. What matters is political will, procurement culture, so systems and structures. This will influence (and bring) all the others in line.