Honestly, they should make it so you can earn new guns as you prestige.
Nothing that would be overly powerful and leave the dudes who are behind at a great disadvantage, but adding new guns, or even new camo's and or attachments for your weapons would probably encourage more people to prestige.
But again, they would really have to make it so there isn't any type of significant advantage for people who are ahead of others, or else there would be a lot of whining.
There should be more incentives to prestige in call of duty games, I mean sure, you get more custom class slots but the average joe only uses 3 slots frequently in the game.
Why does the world spin round? Why do birds fly in the sky?