I'd respectfully submit that there is no 'motivator' that will push people to up into a horde. Perks are my main score motivator, so I tend to fly higher ENY stuff when I'm thinking about it, but I hop around from plane to plane and planeset to planeset depending on my whims. Often, I fly the week's FSO ride or possible rides before Friday.
However, even though I like perks, that's not what makes me up and head toward a horde. I do it because I find it fun, and I know when I get there, there will be enemy planes to kill. I assume people who fly in the horde in low ENY planes aren't there for the perks or the score, they're there to take the base. If you got 10 fighter perks for getting shot down in a goon, it wouldn't make me fly troops any more often because I don't find that part of the game fun. I believe most people are similar. They do what they find fun, or what they think the 'objective' is.
I think history shows in the game that a side that concentrates on defense winds up losing the war fairly consistently. I believe this is why people tend to horde roll, because it is the most effective way to win, in the absence of a truly coordinated offensive/defensive strategy.
Wiley.