Why are so many transponders capable of Mode C but not enabled? The £600 odd that most 'avionics' engineers' will charge has a bit to do with it.
The encoder is abot £130, the wiring loom is a bit fidddly but nothing your average DYIer could n't manage. The static connection very easy. But try and do it on a c of A aircraft and it all gets expensive.
Certainly you would have thought combined transponders and encoders would be childs play, but with the natural conservatism of the US manufacturers it has yet to happen (why bother when you can keep selling the same old rubbish.
However at least one to my knowledge does have it - the new Filser Mode S boxes have an encoder built-in and apparently very straighforward installation - i.e. live, earth, aerial, static. Progress at last!