Because they are brainless morons......If your talking about the Oprah and Jerry Springer type programmes then the audiences are full of slack jawed, inbred white trailer trash who think that a retort like this is the height of rapier like wit.
It's been taken further... there are lists a mile long of "Yo' mama..." jokes on the Internet. Sample, at random (though I had to look for a realtively clean one): "Yo mama's so old, she sat next to Jesus in third grade". nb. I never said the jokes were funny.
[keep losing this blimmin answer)
It is a cause of many fights in inner london schools too! 'Your Mama' (as has been pointed out) is short for 'Your Mama is... a prostitute, easy, ugly' etc. Basically, it is considered a huge taboo - you just *don't* disrespect your (or anyone elses) family. Insulting or cussing someone's mum (by calling them a prostitute for example) therefore breaks that taboo, hence the *ultimate* insult - and the fights!
This is an insult that started out as an insult in the Projects of the USA. It is called 'Doing the dozens' and the aim is to come up with the most humerous insult of the other persons mama. It can be done on a very informal way or lead to a big fight depending on time/place/context. It has spread to the wider community.
Yo Mama so old, she owes Moses money. Yo mama's farts are given names by the weathermen. Yo mama so big, NASA orbits round her head. It makes a change from jokes about nations or racist remarks. As the previous answer: family first.
Actually it's derived from the Spanish / Latin American refrain: "Y tu mama (madre) tambien" - in other words "And your mother too / also". So that if someone insults you, you're essentially directing the same insult immediately back at that person's mother. Clearly it's intended as the ultimate insult (much like the American motherf****er) but in actuality it's merely the product of a rather juvenile and mysoginistic brain. Afterall mothers are the lifegivers and no ugly slur can ever detract from the fact that mothers are all-powerful because none of us would be here without them. Besides, mysoginists generally rely on their mothers much more than the average, well-rounded human being.