If you were to mandate complex systems like this you would effectively destroy GA, Joe Pilot at 500 AGL doesn't need TCAS and never will... TCAS itself is an aid to see and avoid, you can't maintain TCAS separation only visual separation. TCAS itself was deigned and envisioned not to replace see and avoid but to supplant it due to increased closure rates as aircraft got faster and faster, hence why it's use is focused around places where one might expect to find a jet aircraft.
And even TCAS is by no means a be all end all... DHL vs. Bashkirian, Golo vs. the Embraer Business Jet, Saudi vs. Kazakhstan to name a few, technology is not the answer, flying the wrong way in a VFR corridor also isn't the answer, flying the right way is.