Basically the Eurocrats don't like the American way of doing it, because it is American.
One part of it, the other is that the Eurocrats are control freaks of the worst kind trying to justify their jobs with every single bit of paper they issue.
Plus I reckon there is substantial "industry pressure" within Europe to create business for EASA 145 companies generating the EASA STCs.
Most avionic shops and maintenance organisations are going bonkers with the paperwork. They'd rather sell avionics and put them into airplanes than generate tons of useless paper. The fact that they do have to do all this has cost them millions in projects which are not materializing because the owners are so scared by EASA bureaucracy that they rather forget about it and carry on with their old KX175B's.
I talked to some industry reps of Aspen and Garmin. They both told me that EASA has made their business so much "unfun" to do that given the choice, they'd consider selling to FAA land only. One guy, who had been posted in Calcutta at some time, tells me that the Eurocracy is starting to move into the direction of the notorious Indian paperpile harrassment organisation.
Anyhow, if the current administration in the US get their way, you might see a lot of hassle there as well. There are quite some folks in that administration to whom the EASA way of life suits just fine...