Consider some of the basic steps. Cha cha's standard side basic offers a chance for both parties to "challenge" each other; the rock steps on 2 and 6. The give and take builds up tension like static builds when rubbing a balloon on your head. The syncopation that gives the dance its name allows for an exertion of aerobic energy while the frames of both partners remain connected and close.
And let's bring up that infamous latin motion. Considering both halves of the basic, cha cha allows for 8 hip settles, while rumba only allows for 6.
Even though rumba only allows for six, the timing allows for more movement then the chacha allows. The hips are able to rotate more slowly, putting more emphasis on the movement, making it more dramatic and showing more technique then the cha cha.