Are we talking about converting from pagan to catholic? The conversions were mostly top down with the king and court converting and putting a catholic layer over lingering paganism that lasted underground for generations afterwards. The biggest reason is the King went from being in charge because blood and force to anointed agent of God, making fighting the King the same as fighting god. Then you had the ease of dealing with few bishops rather than dozens of local priests in religious matters. The priests brought in bureaucracy as they were literate and many became clerks for the King allowing greater control and communications. laws were written down and taxes became more formalised and legal agreements such as treaties and land holdings stopped being based memory.