Basically all Eastern European flag carriers serve New York and all Eastern European carriers are getting nice subsidies which make those flights worthwhile.

There is simply not enough demand ( yet ) for carriers to serve WAW, PRG or BUD in addition to the national carriers, only DL sees enough demand and has announced to serve BUD from Kennedy.But DL sees potential in any international market right now and will apply for domestic flights within Cuba shortly...

Seriously though, some Eastern European gateways are really high yield , but are not nearly big enough to justify nonstop TATL service, a fact which is used by OS or LH nicely to get some feed into their TATL services...