I'm pretty sure it's because Ye Olde English was herpa apostrophe derpa flim flam jello pudding.
You sound like my wife. That is way, way over my head.
Thanks for citing something concrete to clear it up, though.
You can look up any OE grammar and see that it's the case but here:
http://en.wikipedia.org/wiki/Old_English_grammar#Strong_nouns. I know that's Wikipedia, but the declensions match what's listed in both my OE grammars (Bright's Old English Grammar, and A Guide to Old English by Campbell) so it is correct, but I can't find any really solid sources for OE grammar online, sorry.
Here's another with the same declensions listed. Either of those books I listed would be a good source, though.
As for the CMS, I don't think there's an online version of that so I can't cite it. I
can cite the NY Times style guide.
Use apostrophes for plurals of abbreviations that have capital letters and periods: M.D.’s, C.P.A.’s. Also use apostrophes for plurals formed from single letters: He received A’s and B’s on his report card. Mind your p’s and q’s.
But do not use apostrophes for plurals of abbreviations without periods, or for plurals formed from figures: TVs, PCs, DVDs; 1990s, 747s, size 7s.
I do not think there are
any online sources for the development of the genitive and plural '-es' into what we have today, but you can check out David Crystal's Cambridge Encyclopedia of the English Language. At some points throughout history both forms appeared with and without the apostrophe but it seems likely (and this is just speculation) that the plural dropped it completely because it is A) a slightly more common grammatical feature, and B) to distinguish the two.
You can see why I didn't cite anything though because citing books on the internet isn't very helpful and makes you a jerk.