Why don't Christians decide to become Pagan?

There is nothing more uniquely personal than one's relationship with the Divine, each of us must find the past that works best for us, there is no "one size fits all" answer. If Christianity is a fulfilling and successful way of life someone why should they feel any need to change? As the saying goes, "If it ain't broke, don't fix it". One could as reasonably ask why you don't decide to become Muslim if someone else finds that more fulfilling than Paganism.