Because not everyone is obliged to like everything? Even then, its more complex then that. Like DC vs Marvel. Comics wise I love them both, but the movies are a completely different thing. Sometimes one thing is much better then another (although what's better is obviously hugely subjective). Nothing wrong with liking one thing but not another similar thing. To be fair, I think Orville will be pretty terrible, I'm just slightly less irritated by Orville then the alternative which is why I took a jab at the other show.
So, I guess in this case its barely even one vs the other, since I think they'll both suck, just one slightly less so. Plus, I think Macfarlane is actually a fan of Star Trek, which means even if his show sucks he's at least coming from a better place then the people making the show which must not be named (which I'm only not naming because of a whim and everyone knows what I'm talking about anyway).