Why, oh why couldn't there have been an episode with Heathcliff and Riff Raff meeting one another? The end credits gag was such a tease!
Because, as good as that sounds, this world has had enough crossovers already.
I think part of the problem may be that both are 'alpha cats': they both enjoy being the guy in charge, the one everyone else is supposed to respect/fear. Put the two together, and they'd naturally butt heads — which raises the question of who should win? While the show was named after Heathcliff, Riff and his gang were the Ensemble Darkhorses. Having Heathcliff win would upset their fans, while having Riff Raff win probably wouldn't go over well with the execs. And an Evasive Fight Thread Episode would feel like a major cop-out to anyone looking forward to the obvious clash... Then there's the fact that such a crossover would only make the contrast between Sonja and Cleo more glaring... Having Hector, Wordsworth and Mungo appear in Heathcliff's side of the show didn't cause as many problems because they were lower on the totem pole already; they weren't a major threat to Heathcliff's position. Even if Hector wanted to be.
According to these insiders who wrote a book about the DiC company, Riff Raff wasn't allowed to appear in any Heathcliff episodes because DiC was trying to create a character that would be their own Heathcliff or Garfield, and there was some concern that if Riff Raff interacted with Heathcliff in any way, the Mc Naught company (owners of Heathcliff) would want ownership of him, so they kept both characters distanced from each other (save for the end credits gag). No explanation for why Hector, Wordsworth, and Mungo were allowed to appear in Heathcliff episodes though.