[stvltvs]

Because U was already used in vi, although that form of undo isn't super helpful IMHO. Both R and r are also already used, so Ctrl-r for redo it is.

[cassepipe]

I found that going into visual mode and going `gU` or `gU` + movement was just as useful to get uppercase and `U` itself could be safely remapped to mimick <C-r>