Given its tech stack, why Mastodon?

So I spent the evening looking at what super simple blog applications were out there now (e.g Blot, Bear, if you know any similar give me a shout) and also started looking at all the microblogging options available (micro.blog, mastodon, pleroma etc). This lined up quite nicely given tonight's events and decided that running my own single user mastodon instance might be a good idea. I then started reading up on a comparison of mastodon and pleroma and how lightweight the latter is in comparison to mastodon which made me read up on mastodon's architecture some more. So my questions are as follows:

- How does Pleroma's language and architecture achieve feature parity with Mastodon without the resource consumption? Are tradeoffs being made or is the Phoenix and Erlang language and tools just that much better than Ruby on Rails?

- Why was Ruby on Rails selected for Mastodon and how has Mastodon become the most popular fediverse offering whilst running on 'old' web tech given that the early adopters were presumably very tech orientated people? Should we not be pushing for the 'next big thing' to be in Rust or Go or something else that's fast and performant?