The problem is that at least for XM (and I beleive it is similar with Sirius, but am not 100% sure), they are pretty much maxed on bandwidth. Each year when they do the holiday channels, they have to temporarily delete an equal number of normal channels to free up transponder space.
So, they can't just add in 5 or 10 or however many channels onto the XM sats without removing that many channels that they offer now.
Since the only satellites that a current XM radio is capable of using are the XM and vice versa for Sirius, they can't just have the Sirius people and the XM people tune to the same Jazz (or whatver) channel to free up space either, until most customers have dual band radios.
Lets say they decide that the XM Jazz and Dance channels are better and they delete the Sirus versions and they decide that the Sirius decades channels and delete the XM versions. They can certainly broadcast the same actual channel to both and maybe save a little money due to fewer DJ's and equipment, but they still have to send that one feed out to both satellites or some customers will not get them.
Now, in the future, anyone with a dual band radio can get everything, but anyone with an original XM or Sirius radio can still only see the channels being broadcst from the sats as they are right now. So, maybe 7 years from now, they might start truly start eliminating duplicates but if they do it now, they will make lots of people mad.