Why is it?

I know this is a tough one to understand, please allow me to explain, in the film deliverance there was these people who through years of interbreeding had become weird sexually and they routinely shagged their sisters and daughters. So the joke is that they all had the same DNA. and they couldn't afford a dentist because they were all piss poor.

It is quite simple when explained isn't it?

In the states we substitute "deliverance" with "Arkansas". Achieves the same effect.