You’re oversimplifying a bit. Lithuania stayed pagan for a long time because of many different interest groups, both foreign and domestic.

Lithuania conquered a lot of Orthodox Rus lands and the Rus feared that they will be forced to become Catholic, for example. Also, there were few strong Catholic states in the region that could make strong allies.

Poland made Lithuania convert to Catholicism by accepting their grand duke Jogaila to become their king (by marrying their princess Jadwig), thus making him de jure leader of both Poland and Lithuania. For the Polish, it was an attempt to reduce Austrian influence on their state.