The rack has no screens, so no need to drive VGA, it's true.

Probably the best history on the choice of going with ARM comes from here: https://news.ycombinator.com/item?id=28989138

The internal RFD on the SP's design still talks about choosing RISC-V, and I don't think (or am simply bad at using search) that the move ended up being in an RFD.