I think it is a combination of items # 2, 3, 4*, 5, and 7* from your list, with a heavy dose of #3. I put an asterisk on #4, in that I don't think it is conspiracy theory related. I think 2015-16 was the culmination of a long stretch of "freedom of movement" rules changes that resulted in officials calling A LOT of defensive fouls on drives. That played perfectly into Allen's bull-rush style, and refs rewarded him handsomely. The refs have dialed it back a bit in the last two years, perhaps realizing that they were a bit too trigger-happy in favor of the offense in 2015-16. As for the asterisk on 7, I think he's realizing the abuse that he was taking from all the drives and the reduction in calls he got last year, so he has cut back some on that. I think he's also tried to expand his game, and that attack dog driving mentality may have lessened a bit as a result.
But I think the biggest single factor is that the lane is way more crowded this year, which limits his opportunities. For a guy who isn't the most nuanced of ballhandlers, those daring drives to the hoop are now into a mass of bodies in the paint. It's just different when Duke had 5 guys all on the perimeter (with Plumlee dragging his man outside to set screens) than when there are 3 defenders in the paint (2 bigs and the guard who doesn't have to stay close to Duval).
I do think also teams are scouting him better, and are better prepared for his bull-rush driving style. And as a result, those angles aren't as readily available.