Mostly, because there is a difference between saying "I don't like the game and here's why..." and "This game sucks and you suck because you like it". For some reason, people have a tendency to forget that fun is subjective assume that if they didn't like a particular game and did not have fun with it, nobody else should like it and have fun with it, which tends to provoke responses from people with opposite experiences.Also, this does not apply to just TOR, it's pretty much any game (browse the WoW forums a bit, you'll see countless examples).
There has been a study on this, well peoples reactions to things they buy.
It boils down to a person wanting their product they spent money on to be the best, their choice to be the best choice, nothing bad about their prduct.
I think this came from a study on fanboys and why people feel the need to brag about their product or bash on others about how much better their product is, and it comes down to people wanting their product to be the best so bad they don't see other peoples choices because only their choice can exist. (Because otherwise they would have made the wrong one and they would feel bad.)
So when someone spend their money on SWTOR they need to go and brag about it, tell everyone how amazing it is to make their choice seem like the right one and to make themselves feel better.
I guess I'm usually more curious as to why people feel the need to chime in to inform others that they don't like something (especially when that's ALL they say).
"Man, I could really go for a peanut butter sandwich." "I know, right?" "I don't like peanut butter." "..." "...okay?" "Yeah, I just think it sucks." "Uh, thanks for letting us know. We'll be over here enjoying our peanut butter sandwiches. Without you." "You shouldn't, peanut butter sucks." "Seriously man, we get it. Move on."
If you don't like something, don't use/support it. If you want to like something but have a problem with it, be specific if you can, and if at all possible try to offer a solution. Not doing so strikes me as lazy, and is just a way to pointlessly complain and serves to do nothing but rain on someone else's parade.
I don't like peanut butter, btw, due to it's attempts to turn me dead. I don't suggest or imply that others shouldn't like it, however, which is what many gamers (or game followers) do. I manage to turn my distaste for the stuff into an entertaining joke, while at the same time using it to get my dog to take his pills when sick. It's useful stuff, peanut butter. Wow. Ok. Enought tangent.
What's a compelling reason? Is "I don't like the space setting" not a valid reason? Is "I find most of the classes to be pretty uninteresting" not a valid reason? Is "The quest dialogue system feels really tedious to me" not a valid reason? Is "I never cared much for the Star Wars lore and universe, so I don't find an emotional attachment to the plot or setting" not a valid reason?
Why does someone need a "compelling reason" not to like something. I don't like asparagus. Sure, it's good for me, but I just don't like the way it tastes.
I don't like the way SW:TOR tastes, metaphorically. I have nothing against it, it's just not for me. Why do I have to defend my opinion when I just casually mention that I don't care for it. It's not like I'm going around shouting "TOR SUCKS" to all my friends who play it.
It's plain and simple fanboy-ism. People get defensive about the stuff they care about or like so it's just a natural response, and the anonymity of the internet doesn;t help.