Ah yeah, probably because Twitter was originally written in Ruby on Rails and it's always nice to do exercises that feel like something real. But yeah, making things actually scale is pretty difficult and they felt they had to move from RoR to something else (Scala in this instance).

Of course, Elon Musk probably fired everyone responsible for making it scale properly, so at this point, stopping the growth of Twitter is probably good, because otherwise it might run into issues again.