Probably because there have been more high-profile stories of companies migrating off of Ruby on Rails to something else (e.g. Java, Go, etc) rather than vice-versa of migrating into it.
Gitlab is, AFAIK, nowhere near the scale of Github. Shopify IDK, but I'm fairly certain their type of usage is very different. Basecamp is also of another scale, and certainly a very different usage/performance type.