Wow, didn't expect Civ5 to get trounced quite that badly. Guess it still needs several expansions and patches before it's worth getting.
I'd put it as more a reflection of how positively Civ4 (in particular) is seen than of how Civ5 is seen. I mean, even those that actually do quite like Civ5 (such as myself) aren't necessarily going to vote it as superior to Civ4.