The Capcom vs. SNK games worked because SNK's fighting games actually have somewhat similar gameplay to Street Fighter. Tekken and Street Fighter are about as different as fighting games can be.
Of course it's pure fan service, but they could have made it Namco x Capcom(and I mean a real Namco x Capcom fighting game) and done even better on fan service. People love to see Tron Bonne fighting Dig Dug.