The Show Is Similar to Another Show on the Air
Two shows with the same premise have a difficult time co-existing. By far the most famous example of this is Studio 60 On the Sunset Strip coming out the same season as 30 Rock. We remember that scenario best because then-unknown Tina Fey slew Goliath by besting Mr. West Wing and Coke, Aaron Sorkin.
A portion of the responsibility for Wonderfalls' quick demise has to be laid at the feet of Joan of Arcadia. Sometimes, neither show makes it out of that gate. 2002's Do Over and That Was Then were two identical shows that helped precipitate each other's demise.
This rule does not apply to police and medical procedurals, unfortunately.