D had a lot of momentum in the beginning, but the license around the compiler scared away some people, then there was the whole standard library split with phobos and tango (irrelevant now and has been for over a decade), and from my understanding in the beginning (I wasn't around during that time) Walter added features by request as there wasn't a DIP process yet (D improvement proposals), and some people felt it was unfair. These have all been rectified.
D had a lot of momentum in the beginning, but the license around the compiler scared away some people, then there was the whole standard library split with phobos and tango (irrelevant now and has been for over a decade), and from my understanding in the beginning (I wasn't around during that time) Walter added features by request as there wasn't a DIP process yet (D improvement proposals), and some people felt it was unfair. These have all been rectified.