Nothing was wrong with Futurama, the error lies in the brain of the FOX executives, or the lack of same.
...but seriously. During the 3rd and 4th airing season, pre-emption was more the norm, than the exception. Thanks in large to FOX's "brilliant" decision to place Futurama in the 7pm timeslot.
As a result FOX suddenly found themself with more than 20 unaired episodes, and told Matt Groening that they wouldn't be needing any new episodes made in 2002, and the show was put on hiatus. A fine word meaning "FOX might order new episodes later, but don't count on it". Problem is that the production people couldn't wait around for FOX to make up their mind, and subsequently went looking for other work to put food on the table.
Maybe FOX will order new episodes in the future, but getting the original cast back together would be very difficult.
FOX knew this, and will probably use it as an excuse to cancel the show for good later this year.
Probably, not for certain, but draw your own conclusions. FOX never understood Futurama, and they never accepted the fact that Matt had creative control, and as a consequence FOX couldn't change the concept. FOX never gave Futurama the support´it deserved and needed, and will probably succeed in killing the show, after trying for so many years.
In conclusion; it's all FOX's fault.