I don't know why you would consider JS/TS backends to be niche - they are fairly mainstream by this time. You also need to distinguish between larger framewor

I don't know why you would consider JS/TS backends to be niche - they are fairly mainstream by this time. You also need to distinguish between larger frameworks and compile-to-js/wasm languages here. As for the rest, I think the bigger reason they don't get traction is that one one hand they don't work well for incremental adoption and on the other they introduction friction when integrating with the wider ecosystem. This hampers adoption for both new and established codebases. If I am starting a greenfield project from scratch, I am likely working with uncertain/changing requirements and need to move fast. If I choose something like Blazor/Vaadin etc., I am not sure how much effort will be needed if I need to integrate a third party Gantt component, or a month calendar view, or a leaflet map etc. in future. Unless I am already super-sold on said language/tech, it is likely not the best use of my time to do a comprehensive evaluation right now because I don't entirely know the scope of the project - I'd rather want to spend that time building something minimal that I can push out and get some user feedback. But when the need arises I don't want to get locked into a scenario where suddenly I need to spend a few days dedicatedly writing a custom integration or manually declare types for a complex third party library. So I end up writing a TS SPA because every notable UI library at this point is known to work well with it. In contrast, if I am evolving a brownfield project which already has quite a bit of legacy, I am constrained by the set of choices already made in past. Eg. I am likely not going to introduce a C# layer in a large java application to take advantage of blazor. It makes sense only if I have a C# app, and the frontend developers of said app (who may or may not be same as the backend developers) are equally enthusiastic about C#. Even if I have multiple microservices and each of them can use whatever tech the maintainers of said service want, in order to use Blazor the team still has to be enthusiastic about adopting not just Blazor and C# but also the wider C# ecosystem including ORMs, caching, logging libraries etc. and all of that ends up being a substantial learning curve for a team with deadlines. Each of these constraints funnels down the developer subset likely to adopt this tech further down and down. So all in all, adopting a large fullstack framework is not just a matter of willingness to learn - it is also about how much work I need to do for integrating third party libraries, how much does my write-compile-preview feedback time suffers, how may I have to adjust my dependency system, what other choices the said framework imposes upon me etc. In contrast, if I want to try out a small self-contained reusable library/component it is much easier to incrementally adopt or experiment with in a new or old project.