IO-540
It's not the equipment manufacturer that applies for the STC -it's an EASA Part 21J DOA.
When the major American manufacturers of certified glass cockpit kit applied to get their FAA AML STC's EASA validated (some have over 200 aircraft on a single FAA STC), EASA told them politely where to go. The big problem now is that if you take a product like the Aspen EFD1000, with a retail price of under $10K, but it's going to cost that much again to certify in each aircraft type, then clearly Part 21J DOA's are only going to pitch STC applications at those aircraft types with high potential for upgrades - Pipers/Cessnas etc, and the lower-number aircraft aren't going to get a look-in. I've heard it can easily take upwards of 120 hours of paperwork for the DOA to create a typical STC for the current generation glass systems. Can you see many DOA's investing in this without a pretty good certainty of getting a return on their investment?
One DOA in the UK got EASA agreement for 7 aircraft types on a single STC, but then EASA changed their mind because they weren't going to get all of their fees - €1000 per aircraft type with each on separate STC application. It seems cost over-rules the safety benefit!