Also because if they have to be fans of the most boring fighter so should everyone else.
Rock music fan: classical music is boring.
Classical music fan: rock music is boring.
Adventure movie fan: drama movies are boring.
Drama movie fan: adventure movies are boring.
North American football fan: soccer is boring.
Soccer fan: North American football is boring.
Elementary school kids think that everyone should agree on what's boring and what's not - everyone should like the same music, the same movies, the same sport. Adults are supposed to know better.
If someone is GSP fan then by definition they don't think he's boring, any more than a soccer fan thinks soccer is boring, or a classical music fan thinks classical music is boring. This is such an obvious truth to any adult that I can't help but think half of Sherdog (the ones that think everyone should agree on which fighters are boring and which are entertaining) are still in grade six.
Personally I find Fedor to be the most entertaining fighter, but I'm old enough to know that just because I think so doesn't mean that everyone else should think so too. If you want to know how many people think a particular fighter is entertaining, look at their PPV numbers. If they have high PPV numbers then a lot of people think they're entertaining enough to spend $50 to watch, if they have low PPV numbers then not many think they're entertaining to spend $50 to watch.
And looking at PPV numbers tells you one other thing: the potential PPV market in North America is about 100 million (the approx number of households). Given that even the biggest PPV draws are barely more than 1% of that, it shows that 99% of North America finds every MMA fighter boring.