Yes, I think the Europeans underestimate the difficulties of Shengen, both real and perceived.
Me, for instance, have sailed in turkey a number of times since 1993 and have little or no interest in going back there as I wish to explore the western med. But doing the western med means difficulties and enforced moves to turkey, or to other north african countries that have all have difficulties, revolutions etc in the last few years. Even Morocco.
Plus theres a simple fact that many Americans just do not want to visit countries where they are hated. Go find a country that loves burning American flags and stay there for 3 months? Not on your nellie!
So folks have to get full visas so they dont rely on the shengen visa... Thats much more paperwork and planning.
You must remember that the caribbean is a blow in, blow out place where every country gives visa on arrival for US citizens, and the next island country is only 20 to 80 miles away. From the BVIs to Trinidad is 450 nms where Nice in the South of France to Marmaris in Turkey is 1,200 miles through some pretty tough waters.
Remember Turkey boarders Syria. OK its a long way away from the cruising areas, but it puts some people off.
Croatia isnt a viable alternative either, especially for wintering, and its joing Shengen next year anyway.
Finally, the Med for a retired couple sailing from the USA is a multiple year cruise so if Europe wants the tourism benefit it needs to change the law for cruisers to become as attractive as the Caribbean.
Mark