Most birds and primates live high above - in the air or in trees - and those that don“t walk on two feet. They all need good eyes and vocal communication, traveling long distances and communication requires complex brains. Complex brains could favour evolution towards monogamy: Mating behaviour (finding a partner and keeping him/her is a challenge - for all creatures). It could also be the other way round: Two parents can foster their offspring better, so there is evolutionary pressure towards higher intelligence.
That is convergent evolution of four traits in parallel between primates and birds.