There are some very long and technical explanations of the workings of Mode A, C, and S on the various techie websites. It's fascinating stuff if you like that sort of thing.
Basically, there is a finite maximum number of Mode A/C transponders that can be handled by any radar head, before it all starts to go silly. Mode S allows the radar unit to select which aircraft to interrogate, so cuts down unnecessary/unwanted replies and hence the garbling and other problems that go with Mode A/C.
The problem is that Mode S, which is now becoming mandatory around Europe, is early-1980s technology and already obsolescent. The later generation is ADS-B which is being introduced in the USA. No doubt we'll all buy and install our mandatory Mode S transponders, then a few years later be compelled to spring for ADS-B units.