In my opinion, there are two main factors driving this:
- Lower spec devices have a hard cap on overall code complexity imposed both by available ROM or flash constraints (big projects literally won't fit on the chip), and by time constraints (if your chip is running at <= XX MHz when it's active, you don't have time to run very many functions between events or interrupts). Most projects for these devices won't grow to the point where you really need the code organization benefits that C++ provides.
- It's a lot less effort to port or implement a C toolchain for your chip than it is to port or implement a toolchain for a more complex language like C++, Rust, or even Ada. It's not just the compiler - you also have to have a working standard library (even if some functions are just stubs), an interactive debugger, and integrations with IDEs (if you already provide that for C). All that software engineering is expensive, and you have a much smaller market of developers to amortize that cost over.
These constraints aren't as binding for high-production, higher-spec devices like popular families of ARM Cortex-M chips, so usage of C++ seems to be relatively more popular for those devices. Even then, embedded work normally requires more of a "C with classes" or "C plus the std::algorithm library" approach, which is different from C++ projects you'd see that target servers.