I think one of the problems is due to the way we have a larger cultural over-view than we have in the past and the how humour, which may stand the test of time, doesn't always bear repeating or being plagarised. Our popular culture is now partly driven by a nostalgia industry in a way that it never has been before.
Whilst there's always been art revivals &c., I think what we experience and relive now through 'Weren't the 1970s Fucking Great? Part XII' and other easy television programming is very different. Popular culture has allowed us to be curators and archivists of our own media libraries in a way that even the very rich couldn't have imagined a 100 years ago. This affords us a greater awareness of what's gone on before in pop culture and an intimacy with the material that would have only been available to the obsessive collector.
Perhaps due to the availability of so much in our society and the way society has gone in general, we're also more cynical, scathing and dismissive of many things too: cue post-modern theories on the worth of mass-produced commodities &c. I think ultimately, we've seen it all before when it comes to humour. In the past, because we weren't capable of having our own media library of old television series, old radio, old books &c in the way that we are now - and weren't exposed to amount of broadcasted repeats as we are now - we had nothing really tangible with which to compare the past and the present. We just relied on memory to compare our humour whites in the daylight test.
All that's left is to push boundaries even further or wait for new technologies to enable us to deliver old formats and formulae in new packaging. For example, I think Brass Eye and the Day Today were the funniest things I've ever seen on TV but they were hardly new. Morris et al just pushed it a little bit further and were able to create some visual gags with technology that wasn't available in the past with idents &c. I can't see how this will change now, all the bases have been previously covered and, worse, we all now this as the evidence is sat on all our shelves.
* may be total bullshit face *