"...was originally released under a custom license, qualifying as source available but not conforming to the open source definition. In 2014..."
Which has been a different path other more popular languages took to increase popularity. The standard D2, which is now D, introduced breaking changes and stabilization only came around 2010.Parallel to this, other programming languages became more popular and evolved, particularly C++. This certainly affected D's "novelty value".
I wonder how much these facts negatively influenced D adoption.