Personally, I don't understand how Amtrak/ LIRR/ MNR have had diesel /3rd rail dual mode engines for decades, but an diesel/ catenary dual mode engine is new, untested, and revolutionary.
Well part of it is simply the fact that there isn't a huge demand for such a locomotive. Amtrak doesn't absolutely need one, which leaves only NJT really having a need for such an engine here in the US.
Then, and PRR60 or others may correct me, but it's my understanding that one needs a much bigger transformer to convert 11KV into traction power vs. converting 700V into traction power. And one of the biggest components of a transformer is iron, which is very heavy. Need a bigger transformer? Then you need more iron and more space. An engine has both a space limitation and a weight limitation.
So trying to figure out how to shoehorn both a transformer and a diesel engine into the body of the engine and not going over the weight limits isn't exactly easy. Even the P32 AC-DM's have a smaller diesel engine than the P42 because of the extra room and weight needed for converting third rail power. The P32's have a 12 cylinder engine, while the P40/42's have a 16 cylinder engine.