I think it's a problem of "not being the tech of choice". Obscure languages have a higher cost in that you can't just out-source the work of training people to the rest of the industry as easily.
But you'll also get issues with staffing and getting people interested in working in mainstream languages that are less cool than they used to be, frameworks that are on older versions and hard to migrate, deployed through systems that aren't as nice on your resume than newer ones, or on platforms that aren't seen as positively.
I don't have a very clear answer to give about why Erlang specifically wasn't seen positively. The VP of Eng at the time (now the github CTO) saw Erlang very positively (https://twitter.com/jasoncwarner/status/1287383578435780608) but I know that some specific product people didn't like it, and so on. To some extent a lot of the work pushing us aside was just done by very eager Go developers who just started doing work on replacing our components with new ones on the other side of the org, and then propagating that elsewhere.
Whether the roadmap or other policies ended up kneecapping our team on purpose or accidentally is not something I can actually know. I kept pushing for years to improve things for our team, but at some point I got tired and left for a different sort of role.
But you'll also get issues with staffing and getting people interested in working in mainstream languages that are less cool than they used to be, frameworks that are on older versions and hard to migrate, deployed through systems that aren't as nice on your resume than newer ones, or on platforms that aren't seen as positively.
I don't have a very clear answer to give about why Erlang specifically wasn't seen positively. The VP of Eng at the time (now the github CTO) saw Erlang very positively (https://twitter.com/jasoncwarner/status/1287383578435780608) but I know that some specific product people didn't like it, and so on. To some extent a lot of the work pushing us aside was just done by very eager Go developers who just started doing work on replacing our components with new ones on the other side of the org, and then propagating that elsewhere.
Whether the roadmap or other policies ended up kneecapping our team on purpose or accidentally is not something I can actually know. I kept pushing for years to improve things for our team, but at some point I got tired and left for a different sort of role.