Because if it is bad, pretending it's great is doing everyone except Bethesda a disservice. Early on, the hype and fanboyism were relentless, they pumped it up as a "huge game" and their "most polished" or some such shit... and it just isn't really. They have had ages and a lot of capital to try to make it a good game, but a lot of people aren't convinced that it is. Let alone as good as the massive marketing pushes ahead of release would have you believe.

"Remember: they're still Bethesda", "remember the Skyrim hype?" etc was met with denial, eye-rolling and condescension, so the temptation to go "told you so" is pretty strong when proven right five minutes later.

Since then, their PR goons have been going around passive-aggressively sniping at reviews and explaining e.g. that ackchyually the dullness is intentional, and what do gamers know about games anyway (therefore it's actually good and it's the players that are wrong). I didn't think that was particularly classy, either.

Some people still like it for whatever reason, but there are also tons of valid reasons not to.

In short: the backlash against all of this is pretty predictable, and is being shared in a community where it's fairly on-topic.