I think that people who use this line of reasoning do so because, ultimately, they couldn't be bothered to celebrate Christ's Resurrection by going to church. They're too lazy and they use this "pagan holiday" argument to justify staying home and watching TV instead.
Typically its a skeptic’s/athiest’s/neo-pagan’s/cultist’s argument that is picked up, oddly enough, by some Christian fundamentalists who like to think all of the rest of Christianity is out of step but them....