I've joined RethinkDB just a couple of months ago, so I might not have all the historical facts right, but here is what I know.
In a previous incarnation RethinkDB was a highly optimized storage engine for SSDs implemented in C++ to be able to take full advantage of both low level SSD and kernel access.
The current distributed engine was built on top on this storage engine and I think it only made sense to continue with C++.
Originally, RethinkDB was to be a MySQL storage engine, which made C++ the natural choice.
They pivoted away from MySQL after my short stint in the beginning so I can't speak to why the storage code was kept (though I can't imagine it's because my code was so great they couldn't bear to throw it away).
Storage people tend to stick close to the metal, in general. This means C or C++ in most cases, for better or for worse.
