I'll hold my hand up and say I think my previous answer was wrong.
It was on the right track, but in XのではなくY, the ではなく applies to X, not to Y. This makes sense, because ではなく contains a topic marker, and the topic can't very well come
after that marker, can it?
So XのではなくY means "Y rather than X", not "X rather than Y" as I previously said.
I think the の, rather than making X an adjective, has a similar effect to when it's used in のです: that is, it softens the sentence. ではない, after all, is technically the negative of である, which is itself a slightly more literary way of saying です.
To put it another way, XのではなくY means functionally the same thing as Xではなくて、Y.
(By the way, even Google Translate gets this wrong: If you try to translate "Like to play tennis in the spring rather than the summer" it gives you 春ではなく夏にテニスをするのが好きです, which actually means "I like to play tennis in the summer rather than the spring", which you can confirm just by asking Google Translate to translate its Japanese output back into English.)
小狼