Unfortunatly I dont think you will get a straight answer to your question.
My guess would be, part of the reason is because YEC creationist groups often say that Evolution needs "time" to work. Thus, take away time, and evolution falls apart. Believing what Hovind has said, "Times is the evolutionists god" (or something like that). So by trying to limit the age of the earth, they think they are making a blow against evolution, kinda like and attempt to take out evolutions Legs. They then seem to falsely think that if they limit time for evolution, that the only possible choice for everything is creationism.
Its also a way to keep their followers away from "secular science." The farther they can keep them away from real science, the easier it is to feed them creationist data. By trying to show we have dated the earth wrong, they are trying to show that science should never be trusted. If they were to concede to science, I think they fear a slippery slope (which they are on anyway, they just dont realize it), where one might say "well, if science can date the earth correctly, maybe there is some truth to this Evolution thing."
But thats just a guess.