1) Different dietary requirements. Guinea pigs can not make their own vitamin c and without vitamin c they get scurvy. They get most of the vit c they need from vegetables so as a result there is a greater emphasis on veggies in a guinea pigs diet.
2) Rabbits have strong hind legs. Even an accidental kick can cause internal damage and even death.
3) Rabbits prefer multiple levels. Guinea pigs are primarily ground dwellers.
4) They are two totally different species, so can not communicate. It would be like sticking a human with an ape. Whilst maybe similar, you would be very lonely not having someone to talk to.
5) Rabbits can carry something called bordetella which can be transferred to guinea pigs. In guinea pigs it is deadly. (It might be the other way around, I can't remember who transfers it to who).