I would guess it has to do with the tedious Family Guy bashing every week.
I mean, it really is every week now. If the show just doesn't entertain you anymore, wait until 9:30 and turn on American Dad. There are obviously a wide amount of people that disagree with the whole "This was the worst episode of Family Guy ever" shtick.
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The episode was okay, but had way too many flashback gags and pop culture gags.
It simply wasn't that funny.