Lolicon is obvious, it's simulated kiddy porn for sick fucks what want to fuck little girls. (No judgment.
) But shotacon... the vast majority of it I've seen (and as a regular nhentai user, I've seen *a lot*, the last few years it's been taking over the hentai manga/doujinshi scene to nearly the same degree as NTR) is still super male-gaze-y. Like, yeah, there's a little boy there, but the focus is still very much on the women. It feels like, whereas the fantasy being serviced with lolicon is fucking little girls, the fantasy being serviced with shotacon is fucking grown women *as a little boy*, and I just... can't wrap my head around why that's appealing to anyone (let alone apparently to half of Japan).
It's catching up with NTR? Well, as someone who generally feels a vigorous violent rage and a loss of all lust at most NTR, and as shota feels more fantastical than (far too believable) NTR, I'll count that as a win.
As someone not turned off by shota, I've seen 4 draws/attractions that address the pattern you seem to be picking up on:
1st - for the male view the smaller male partner (remember the first shota was a guy with dwarfism) takes up less room this is why you commonly see a shota with a phallus of avg or above avg adult size. The appeal is firstly that the character doesn't block the view of the female. This is the type of shota viewer who might be horrified by a realistic depiction of a male child abused.
2nd - similar to popularity of the rape fantasy, is that of men who feel guilty of their sexual desires, their fantasy is to have the power and responsibility taken from them. This shota viewer may find other stories of female sexual aggression appealing, and the shota provides a certain verisimilitude that can only otherwise be found if the female drugs the male, immobilizes him, or holds him at weapon point.
3rd - as pornography has become accessible to younger and younger audiences, males first sexual experiences are no longer a few occasion with the rare dirty magazine found behind a proverbial locker, or fumbling around with willing girlfriend, but dozens or even hundreds of wack-sessions with a host of pictures and videos of adult women. This can form a sexual identity where the male is far more comfortable seeing himself as the smaller partner and younger member. This shota viewer is likely attracted to the shota harem genre.
4th - the fantasy of the challenge, may be related to kink for amazonian body type. The shota generally has to over come a greater difficulty. This type of shota view generally hates stories of "pity" sex and prefers the shotadom.
Bonus - not as related to your observations but there is a "mother-figure" kink that can be connected, but as that is hardly a novel condition it unlikely connected to the
Of course these are not mutually exclusive to each other or other kinks. I know that as people get more realistic with the shota the less popular it currently is (and personally I hope that trend doesn't change).