At least right now, the answer is probably because they took too much time off during the slow economy. There was a big construction boom for the Millennium with Test Track/DAK/RnR and for
Disneyland 50th with in 2005/2006 with Lights, Motors Action; Expedition Everest, and Soarin (and Cinderelabration).
But, between the Hurricanes in Francis and Charley hitting Orlando in 2004 (just prior to the 50th construction finishing), and the housing meltdown in 2008, Disney decided to cool the construction for a while. They let it go too long and they are now playing catchup.
Universal had their dry spell of construction previous to this (from 2000 - 2010 when WWOHP opened only High in the Sky Seuss Trolly opened at IOA).
Universal had a lot of Irons in the fire, Transformers was being build while site prep was being done for Diagon Alley. Site Prep for Kong was being done during the final construction stages Diagon Alley. Springfield was being rethemed during Transformers/Diagon Alley construction. Cabana Bay has been Constructed and Sapphire Falls is being constructed. City Walk is getting a complete makeover.
But, your point is well taken. At least in the parks, except for Springfield's rethemeing, a project get's 80% done before the next one get started. It's almost like they have a set of construction crews con permanent retainer. "You're done prepping the site for Transformers, OK go over here and prep the site for Diagon Alley, When your done with that go over to IOA and prep the ground for Kong."