It's fast enough, easy enough (being very similar now to TypeScript), versatile enough, well-documented (so LLMs do a great job), broad and well-maintained first party libraries, and the team has over time really focused on improving terseness of the language (pattern matching and switch expressions are really one thing I miss a lot when switching between C# and TS).
EF Core is also easily one of the best ORMs: super mature, stable, well-documented, performant, easy to use, and expressive. Having been in the Node ecosystem for the past year, there's really no comparison for building fast with less papercuts (Prisma, Drizzle, etc. all abound with papercuts).
It's too bad that it seems that many folks I've chatted with have a bad taste from .NET Framework (legacy, Windows only) and may have previously worked in C# when it was Windows only and never gave it another look.
It was actually really good for the time and lightyears ahead of whatever Flash was doing.
But people rather used all kinds of hacks to get Flash working on Linux and OSX rather than use Moonlight.
Which means if you write C#, you'll encounter a ton of devs who come from an enterprise, banking or govt background, who think doing a 4 layer enterprise architecture with DTOs and 5 line classes is the only way you can write a CRUD app, and the worst of all you'll se a ton of people who learned C# in college a decade ago and refuse to learn anything else.
EF is great, but most people use it because they don't have to learn SQL and databases.
Blazor is great, but most people use it because they don't want to learn Frontend dev, and JS frameworks.
"Modern C#" (if we can differentiate that) has a lot of nice amenities for modeling like immutable `record` types and named tuples. I think where EF really shines is that it allows you to model the domain with persistence easily and then use DTOs purely as projections (which is how I use DTOs) into views (e.g. REST API endpoints).
I can't say for the broader ecosystem, but at least in my own use cases, EFC is primarily used for write scenarios and some basic read scenarios. But in almost all of my projects, I end up using CQRS with Dapper on the read side for more complex queries. So I don't think that it's people avoiding SQL; rather it's teams focused on productivity first.
WRT to Blazor, I would not recommend it in place of JS except for internal tooling (tried it at one startup and switched to Vue + Vite). But to be fair, modern FE development in JS is an absolute cluster of complexity.
A big chunk of their strategy at the time was around how to completely own the web. I celebrated every time their attempts failed.
After a while people got tired of doing updates.
And when the front end is C# so is the back end.