I think the lag in certification partly came from NASA's risk assessment of some of SpaceX's plans. NASA increased focus on the risk of micormeteorite damage to the Dragon capsule while it is docked to the ISS, which has required additional shielding. Also NASA wants to see multiple Dragon flights with identical booster hardware before certifying the platform, and so far SpaceX has been iterating on Falcon a lot. Falcon 9 Block 5 is supposed to be the final iteration, so Dragon V2 certification has also been blocked on that.