Why The ionosphere is layered by altitude, and the letters run in order going up: D is lowest (roughly 50-90 km), then E (about 100 km), then F1 and F2 (150 km and up, merging into a single F region at night). Because the D region sits in the densest part of the upper atmosphere, collisions are frequent there and it acts mainly as an absorber of lower HF signals during daylight rather than a reflector.
Watch out The F2 region is the one most often used for long-distance HF skip, so it is the most familiar name, but it is the highest region, not the closest to Earth.
Alphabet order equals altitude order: D, E, F1, F2 from the ground up. D is 'Down low' and 'Daytime absorber'.