Why is it that so many albums don't get released once a label signs an act?
A few things can go wrong. First, it can take a long time for a record company to court an artist, and then another good amount of time while attorneys are going back and forth getting the contract right.
Once the deal is locked in, the label has to schedule the release and the artist have to make the album. From the beginning of courtship to the time an album is slated for release could quite easily be 2 to 4 years. During this time several factors can cause a project to go south.
If the A&R man, that originally championed this act, gets fired or leaves the company, 9 times out of 10, the company will drop that act. Unless the act has had past success, the new A&R guy on board usually has his own vision, or artists he or she is courting, and doesn't get involved.
During the courting/signing and scheduling period, if the project is a group, then it is likely that this group may fall apart for many different reasons. It could be personal, or they decide they can't stand each other.
Quite often artists are not good business people and their lack of grounding gets in the way.
I do see a trend away from this these days as labels are more and more reluctant to get in bed with artists who are losers at life, despite their creative talent.
Finally, the artist might deliver a rotten album. If the album does not contain any marketable songs, IE: something radio can strongly get behind (labels can usually do research to see if a song will be a hit before it is released), then the label will either drop the artist or have them re-record as much of the album as needed. This is where hit producers come in to lessen the odds of disaster. If the project is over budget and a crappy record comes through, usually the artists get dropped as it doesn't make good business sense to keep them on.
Last but not least, the record company can go bankrupt, causing an artist to be stuck in limbo until legal things turn around. This can be the worst scenario of all.