I hope this post isn't pushing the limits of thread zombieism or going way beyond.
It's been years since I read it, but if I recall correctly, Mario Pei's The Story of English mentions the gender conundrum and its loss in English. I don't think he had an explanation, either.
I did find this:I can't link yet: linguistlist[dot]org[slash]issues[slash]8[slash]8-342[dot]html"Richard Krause recalls that 'in Old English (Anglo-Saxon), adjectives agreed with their nouns in gender, number and case'. He also claims that 'Adjectival gender, number and case inflections were probably lost in conjunction with the elimination of noun gender and the accompanying major reduction in case inflections. These major modifications of the English grammatical system probably arose as a result of the Viking settlements and especially the Norman conquest. . . ."
- For Teachers