Why? Read on.
First, there are really only two kinds of sequels out there: Intended sequels and not-intended sequels. The latter of them being the kind where the author already said s/he’s writing a trilogy (or more). And the first is the kind of sequel where the author/producer is overwhelmed by the success and this continues the story; to please the audience and make more money.
(that’s simplified, of course, and authors/producer are surely allowed to change their mind about their work, but let’s just leave it at this, okay?)
Most common in fantasy (and SciFi) literature, the second book in a series is often times intended to give the reader some kind of background and/or depth to the characters before the third one either completes the series and is full of action and conflict. Larger series may contain several of these ‘filler’ books or have them later on (‘A Feast for Crows’ seems to be one of these kinds of books).
The… problem with those books isn’t that they’re poorly written or the information they give is not interesting but the pacing of the series seems to slow down or even stop. Nothing really is happening (which may not even be true) and all you do is wait for the big showdown.
It’s the impatience of the readers which makes those books seem less likeable. They aren’t as interested in the characters as they should be and miss-interpret the calm before the storm as a unnecessary slumber of the story.
So, intended sequels have the pacing problem – or some of the sequels – but what about unintended ones?
Mostly, they lack reason to exist in the first place and suffer from the necessity to prove their value by trying (and failing) to be better/larger/funnier/whatever than the original instalment. Perfect example of this is Pirates of the Caribbean and it’s sequel. The first one was great and Capt. Jacks entrance at the beginning was one of the most hilarious character introductions I’ve seen thus far.
PotC 2-4 were made because of the success of the first movie – and to function it had to be more in every aspect – including Capt. Jack Sparrow. But it didn’t work; for me at least. Jack Sparrow sailing with the Pearl on sand may seem like a cool idea but in reality it’s just way over the top.
A battle between two ships in the eye of a maelstrom with A FREAKING WEDDING ? JUST. NO.
Unintended sequels lack the originality which made the first one successful and so they multiply everything good till it’s just silly instead of re-inventing or adding characters in a meaningful (!) way. Having Keith Richards on screen for twelve seconds may be cool and all that but it doesn’t add value to the character of Capt. Jack Sparrow.
Tom Hollander as Cutler Beckett on the other hand was perfect but couldn’t save the movie at the end…
So, those are just my two cents and there can be said quite a bit more pro and against sequels (or third/fourth/… instalments) but I’ll just leave it at this^^