I think 3 things

1. D had a split similar to python 2 vs 3 early on with having the garbage collector or not (and therefor effectively 2 standard libraries), but unlike python it didn't already have a massive community that was willing to suffer through it.

2. It didn't really have any big backing. Rust having Mozilla backing it for integration with Firefox makes a pretty big difference.

3. D wasn't different enough, it felt much more "this is c++ done better" than it's own language, but unlike c++ where it's mostly a superset of c you couldn't do "c with classes" style migrations