One powerful factor for rulers is that Christianity and the church's seal of approval solidified their basis for kingship and eased dealings with local Christian subjects and with other kingdoms. Formal conversion of the crown was generally preceded by conversion among part of the population as Christian missionaries journeyed through pagan lands, and contact with (and bringing of captives from) Christian territories was fairly commonplace, so the break wasn't wholly abrupt, while as a Christian monarch you now enjoyed the support of church and clergy so long as you didn't seriously misbehave.