There are a lot of causes. One is that the community itself is fractured, in part due to "personalities."
Another is that some of the more modern standards are simply not yet implemented in any of the open Smalltalks (things like JSON-LD etc).
Ultimately, momentum depends on use. One of the Smalltalks needs its "killer framework" that sets it above other options, and demonstrates the true advantages of the environment. Maybe 20 years ago that was Seaside, but the web standards have caught up and surpassed what that framework set out to solve.
I opined on the fediverse that someone should ship a browser that bundles Glamorous Toolkit[1] as a substitute for the traditional devtools that browsers include today. Of course, building a Web browser or even just maintaining a fork of one is hard, which is why it's not something I'm recommending. Releasing something like "Firefox with Glamorous Toolkit", however, is tractable enough that it's something Feenk could do themselves (without any cooperation from Mozilla Corporation, and without needing it).
Another is that some of the more modern standards are simply not yet implemented in any of the open Smalltalks (things like JSON-LD etc).
Ultimately, momentum depends on use. One of the Smalltalks needs its "killer framework" that sets it above other options, and demonstrates the true advantages of the environment. Maybe 20 years ago that was Seaside, but the web standards have caught up and surpassed what that framework set out to solve.