The SDK was probably the biggest culprit. It took almost a year before it reached a stage that was really suitable for more complex 3rd party add-ons and what many would consider to be basic stuff added to it. It’s still very much a work in progress, but it’s a huge improvement now. The SDK will just get better with time.
So this definitely delayed the more complex planes like the PMDG 737 and Fenix Airbus series, and likely prevented them from being built earlier. These planes take a ridiculously long time to build even when everything they need is there. The SDK is the foundation on which all these complex add ons are built. It’s hard to build the building when the foundation isn’t complete.