In fact, didn't OD&D only have 5 alignments? LG, CE, CG, LE and N?
It explains why even now we still have pages and pages for Devils and Demons, not as much for Yugoloths (or other NE extraplanars). LN has modrons and formians and . . . I don't even know. CN has slaads and . . . I don't even know. NG has animal people, because why the heck not. Dragons also tend to the 4 diagonal alignments (especially the Top Ten in the MM).
Similarly, it took a long time before NG paladins became options in core. Even the optional paladin rules in 3rd ed went for the diagonals (Paladin of Freedom, Slaughter, Tyranny). Although Blackguards could be NE so there is that. That said, druids broke out of N and branched out into the orthogonals in third (NE, NG, LN and CN) so there is that. Plus the Incarnates, I suppose. What? *I* like them, ok?
Here's a head canon I haven't tried yet: All 1st generation Gods are alternate reality versions of each other. This does not mean they get along and they were all surprised to see each other as it took the initial creation of the prime material universe for them to appear to each other.