Roblox client does as much as possible in Lua(u). It's somewhat well known that Luau is the language Roblox games are scripted in, but even before you enter a game, for the sake of portability everything is written as much as possible in Luau.
Avatar selection screens, groups, settings etc. all make for a huge number of different screens, so I'm assuming they want React to better coordinate making all that GUI between the huge number of internal devs they must have.
Just my guess, I've personally never used React and am not 100% on whether the Roblox app uses React-lua internally or not.
