Kublai Khan favored Daoist and Buddhist officials over Confucian officials because Daoists and Buddhists: were less anti-foreign.
Confucian officials were known to be strongly opposed to foreign influence, while Daoists and Buddhists were more open to it. This made them better suited to serving under a Mongol ruler like Kublai Khan, who was trying to establish a foreign-ruled dynasty in China. The other reasons you've mentioned, such as not being able to marry, accepting lower pay, and being more authoritarian, might have played a role in the decision-making process but the main reason was their attitude towards foreigners.