Saying that C is better than C++ is dumb, I agree -- you can write just as efficient code in C++ as in C.
The real answer is more often something like "the C++ tools we looked at 10 years ago were terrible, and the libraries had bugs, so we backed off."
Or, even, "the C++ tools available for our particular set of microcontrollers right now are terrible," might be true, for non-ARM, non-AVR microcontrollers. (PIC doesn't have GCC support at all; Propeller just got some limited support; I don't think MSP430 does GCC either, for example.)
Other options for answers include "I learned C, and I'm not going to learn C++, dammit!" which is dumb, and "Python/Ruby/Erlang/Go/JavaScript is much more productive for a programmer than C, so nobody should ever develop in C" which is equally dumb (if not more so) -- Systems programming languages exist for a reason!