Well, according to their SourceForge.net profiles:
VisualBoyAdvance is written in Assembly and C++
ZSNES is written in Assembly and C
ScummVM is written in C++
AdvanceMAME is written in Assembly, C, and C++
Nestopia is written in C++
DOSBox is written in C and C++
I don't think that C or C++ are better suited for emulators, it's just that C is usually credited with being the most well-known language in the world, and it would appear that many of the people graciously making us emulators follow one of the basic formulas: Write the base in C, then use another language to perform more complicated tasks and speed things up.