Why does this happen?One common scenario can occur when a new work becomes an unexpected smash hit and is everywhere. Numerous people check it out, and even those who don't will still recognize it. Then the creator decides to make a sequel... but the fad has largely died out by the time it's released. Many of those who checked out the original to see what the fuss was about didn't like it enough to bother with the sequel. Maybe a significant chunk of the fanbase has moved on too. Thus non-fans only remember the original, but the sequel may still be well-regarded by fans of the franchise.
This reaction can also happen if the sequels themselves are disliked for whatever reason. Maybe people thought they rehashed too much from the original. Maybe they tried to shake things up, but ended up getting rid of what made the original so beloved in the first place. Or maybe they're just bad. This can be related to the above point — maybe the first installment is a hit, but the sequel flops and kills the franchise, so people end up only remembering the original.
And sometimes it's just nostalgia. Fans may view the first instalment through rose-coloured glasses for various reasons: Maybe they grew up with it, so it represents cherished childhood memories, or at least benefited from the fact that children and teens tend to be less picky than adults. Maybe the original was revolutionary in some way, which made it easier to forgive or ignore its flaws. A not-so-revolutionary sequel will then be held to a higher standard, both because people expect the creators to iron out the flaws, and because the audience getting used to the franchise's unique features will make it easier to spot the flaws. Maybe the problem is that the franchise is just gimmicky works that didn't have much else going for them once the novelty wore off (though in these cases, the original may also be viewed poorly in retrospect).
The creators can also reinforce this effect: they notice that the first instalment is the most popular, so they give it (and/or elements from it) more promotion, which gets it even more fans...