Can someone illuminate me as to what are some of the reasons that would make someone style with CSS Grid with a Fallback to Bootstrap?

Considering you are already writing Bootstrap due to compatibility concerns why not just keep using that? I feel like adding another dependency like this would just create a risk of having design differences pop up over time.

Mind you, I don't hate CSS Grid. I really enjoyed learning it, and I look forward to be able to use it in a few years.