Because D cells are obsolete! They're expensive, sometimes not readily available and not very useful outside of a few applications. AA and AAA are very wide spread and inexpensive. The AA in a D adapter will work fine, it just won't last as long. But it's an Eneloop so presumably that's not a huge issue- just charge more often.
“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman