They care the least about the dubia? Not the green bananas? Those things are so tiny, I don't know why they would even be interested in those. Plus they can fly and they're fast. I hate them so much.
Lobster roaches seem to be my chameleons' favorite.
Green bananas get more interest than the dubia. And I don't know- they are smaller, but adults are not really what I'd consider tiny. Especially if you consider the majority of the diet of wild chameleons (as per published scientific studies of a few different species) seems to be flies and bees- neither of which are all that large.
It isn't always about size- I had some huge melleri before that prefered lobsters over the larger dubia or hissers.
I've found many chameleons are kind of like that day to day- smaller prey items seem to be a preference, but then they get very excited sometimes when an occasional larger item comes along- but large item after large item and they have much less interest then.
Also it is possibly about the activity of the roaches. Dubia go inactive and kind of hunker down in the food bowls after only a few minutes for the most part. And because they are larger, it takes a few minutes to eat one or two for a chameleon. So after that, the roaches aren't moving so much. Lobsters tend to be a little more active for longer periods and go for occasional strolls in the bowls after hunkering down.
But when it comes to size- dubia or discoids are pretty close in size. They can be fed the same foods, and are pretty close in nutritional value. Neither can climb smooth surfaces. Their care and breeding is pretty much the same. Both are fairly productive species that are easy to breed.
There isn't anything magical or fantastic that dubia has that discoids don't.