Back when chainmail bikinis were a big thing, you could count on the person buying your fantasy product to, for the most part, be a heterosexual cisgender white tween boy whose parents were working class or wealthier. So big muscles (exactly what your tween wants to be) and scantily clad ladies (exactly what your tween wants to see) get you a sale based on that alone. Anyone who wasn't in that target demo would just have to tolerate the art to get to the content, which wasn't always in line with what the art was promising, anyway. And a lot of people couldn't tolerate the art! But that was OK, for the publishers, since things weren't big enough to care yet. They just wanted to sell a few copies on this weird new thing. Enough tweens in the world for that.
Sorry, but the audience for TSR in those early days,was limited to the pkaces it was sold, which was often hobby shops in college towns as the college student, not teens living with mom and dad.
The art for D&D was less than that of AD&D, which was the one cluttered with full page color art strewn with chainmail bikinis.
D&D had only minor color art and was mostly black and white sketches, so you could not tell if pants or a furkini was being worn half the time inside of a book.
By the late 1970's and even more so in the 1980's, AD&D/D&D were both fairly widely available IME due to the fact most shopping malls, strip malls, etc. had hobby shops and/or bookstores by then. The primary target was definitely male, but the age range was very wide, from 5 or 6 up to mature males of younger and middle ages. It was not just for "tween" boys and not just for young adults likely in college.
As a point, 1E was mostly B/W art, just like D&D. Look through your 1E PHB, DMG, MM, MM 2, FF, DD, OA, WSD, DSG and on and on and on, you'll see B/W art everywhere. Dragon Magazine had a mix, but a lot of B/W art. The only place I recall seeing mostly color was in art books for the game. Most modules had only B/W art. Color was almost exclusively for the
outside. 2E brought more color, but much of theirs went from B/W to blue & white.
And once 2nd Edition comes online and moves into the 90's, we still some chainmail bikinis, but not as much as in the 80's for the most part. TSR and the hobby had expanded, more girls and women were playing, and as color printing became cheaper, more color made it's way into the books.