To name just a few costs that aren't worth it for many businesses:
* A much smaller ecosystem of libraries to draw from.
* Much weaker editor tooling than with more established languages.
* An entirely different paradigm for deployments, monitoring, and everything else that falls under "operations" that may be incompatible with the existing infrastructure in the organization.
* When something does go wrong, using a weird stack means you have less institutional knowledge to lean on and fewer resources from people who've been doing the same thing as you.
* A whole new set of foot guns to dodge and UX problems to solve related to what happens when someone's connection is poor. This has come up repeatedly in discussions of Phoenix LiveView—what you get in reactivity comes at the expense of having to work harder to engineer for spotty connections than you would with a request/response model.
* More difficulty hiring people, and an increased tendency when hiring for selecting people who are really just obsessed with a particular tool and unwilling to see when the situation calls for something else.
There are many more, these are just the ones I can think of without having a concrete application with concrete requirements to analyze. In the end for most apps reactivity is so much a "nice to have" that it's hardly worth sacrificing the stability and predictability of the established option for moderately better support for that one aspect of UX, especially given that you can always add reactivity later if you need to at a slightly higher cost than it would have come at with Erlang.
If reactivity is a core requirement, that's a different story. If it's polish, don't choose your architecture around it.