C++ is (primarily) used where there aren't any other viable alternatives (other than C).

Browser frontends (in general) set not such demands (size, memory, speed guarantees) and its bloat more often comes from things outside of code (images, videos, trackers), so there's freedom to use whatever floats ones boat.

Combined with the ever increasing demands on app complexity, frameworks and born left and right. The strong movement into functional programming in this area has also led the way for other languages (SMLs, Lisps) that better fit that model.