In theory, there are electro-physical reasons as mentioned by Dr Mayhem.
In practice, you will find AC power supplies almost only on larger multi-channel USB interfaces and hardly ever on a one-channel or two-channel device. If you do some research and explore reviews about audio quality of USB interfaces, you will find out that the quality of semi-professional USB-powered audio devices is notably good nowadays. Renowned manufacturers use the same microphone pre-amps as in large multi-channel devices and thus deliver comparable audio quality.
So in theory, there is a difference, but in practice, the only difference is that small one-channel or two-channel devices are USB-powered for mobility reasons without a notable loss of quality.