Historically, the companies with large line of business apps that were interested in using JSIL to run C# in the browser were interested specifically because rewriting their software from scratch in Electron was an expensive and time-consuming undertaking. This made it worthwhile to invest in alternatives that would let them reuse existing code. We're talking massive WPF or Silverlight codebases with decades worth of developer time invested that would all go out the window.

The same logic applies now and it's even more compelling since the tooling and performance are better now with the full LLVM WASM toolchain and more robust C# AOT compilation available, vs the dark ages when people had JSIL, Saltarelle and Bridge to choose from.

I'll also note that every major company I'm aware of that made a C# -> Browser move for their tooling eventually regretted it and started shifting away to something better, partly due to platform churn and partly due to how much was outside of their control. Electron may change the equation there enough to be good enough, we'll see - I don't know of any companies that have taken that route yet.