One of the major problems is the challenge of fitting certified avionics to GA aircraft. With a '60's or 70's S.E.P GA aircraft available for the cost of a family sedan the cost of fitting 8.33 khz radios , ADS/B, Flarm, extended mode transponders, EPIRB etc in a reasonably integrated unit into the panel simply outstrips the cost of the aircraft.
EASA refuses to accept the FAA certification of equipment with tens or even hundreds of thousands of units fitted in the US, requires massive expenditure for certification in a tiny market and requires over the top installation processes.
I'd fit a complete stack to my little ship if it wasn't for the fact that it would double the investment I have in her.
I fly, I look out, a LOT!