Porn is a reflection of male fantasies and realities. Many men, when young, had their first attachment to an older, "dad" type male -- a teacher or coach or scout master. The fantasy often involved the older, wiser, more experienced male seducing the young male, e.g. the young twink. The natural progression of that is that the "dad" would be the "top," the instigator, the one who talks the younger lad into kneeling before him and sucking his cock or bending over and taking it up the ass. The bottom, the twink, is the "cherry" -- he loses his virginity to the older man. Sometimes some roughness is involved, where the young lad might get second thoughts, but "dad" persists and "takes" the twink, who then will be sexually satiated by this experience.
So porn with the older top and twink bottom is simply a reflection of this fantasy/reality, both for young men and for older men who now are in the position of playing the "dad" and fantasize (or actualize) seducing young men and taking their virginity. The inverse of that, with the "dad" as the bottom and twink as top, likely plays into far fewer fantasies. Certainly, some older men love to bottom for young men, but that usually only happens when the bottom, too, is experienced. Twinks, by their nature, are a porn phenomenon somewhat synonymous with inexperience, so the paradigm doesn't quite fit where the experienced older man is seduced and "used" by the inexperienced young lad.
Young gay men today probably often have first sexual experiences that differ from those of us in our forties. In "my day," I think most young men lost their virginity to wiser, older men who knew what was going on, whereas the young lad was still rather confused. Now, with the internet and gay lifestyle being much more common, accepted, and open, young lads aren't so confused as we were. The idea of a male getting or giving a blowjob today, when you're 18, is no surprise. When I was 18, having grown up in a solidly middle class suburb, the idea of a man sucking another man's cock was shocking and subversive. You needed an older man to guide you into the realm of those possibilities.