The whole idea of adding Sledgehammer Games to the team of Activision and Treyarch was to give each developer full 3 years to create their iteration of the Call of Duty franchise. Seems Treyarch thinks that this time was much needed.
In an interview with Game Informer, Dan Bunting (multiplayer director) said that the new gameplay style might not have been possible to achieve in the old two-year development cycle routine,
“In the two-year development cycles, the kinds of risks we're taking right now, we wouldn't be able to do with just two years because it needs so much iteration. We iterated on the first set of multiplayer maps for] a year. That's three maps we were working on and each one was constructed in a different way so that we could learn how the different map shapes and the different ways that you pace combat felt differently with the different moving mechanics as they came in.”
Seems like the new Specialist character system they developed was another feature they might not have been able to pull off otherwise. Bunting says that this is a more fleshed out version of the Pick 10 system they implemented in Black Ops II. Without the additional time, they might not have been able to improve that system any further.