Probably only because that's what he's familiar with. Rails has the most ubiquitous scale problems that large co's keep migrating away from, and new devs are trending away from it. Shopify investing into the JS/TS backend world is just another domino falling of Ruby/Rails descending into Perl-like prevalence.

elixir/phoenix gives you a lot of the same productivity as rails with vastly higher performance. the big benefit to rails these days is the massive ecosystem of drop in gems.