Because it would mean giving up pork & Xmas trees & getting circumcised among other things!!!!!
Pork is not so tasty at all and the Chrismas tree makes only work (The needles have to be cleaned).
Circumcision is done today by real professionals, so the pain is not comparable to old times.
Plus I highly doubt, that this people are devout christians, who have really faith issues.
Perhaps you need to make some kind of invitation.