I think it might be because copyleftists are basically saying that nobody should get paid for their work, and everyone should benefit from everyone's work for free.
That is very nice in a purely intellectual sense, but there's just one little issue : how are the artists who don't get paid manage to pay for their food ? Not to mention rent, heating, and all the rest. Not to mention the hypocrisy of removing revenue for artists when the copyleftists are quite happy being paid for their work.
I personally have no problem with copyleft on software I create in my spare time, but I find it difficult to accept that for software I write for a living because I need money to pay for my bills, and copyleft removes that money source.