ESSENCE OF BHAGATISM
Bhagvatism believes in : action performed attached to its fruit is fetter causing endless of briths while nishakama karma is release and true renunciation, selfless, loving pursuit of personal God is true bhakti which inspires selfless action and service, pursuit of one's varnasharma-dharma i.e. social obligations is one's true dharma, God incarnates Himself from time to time in human or any other form in order to uphold dharma and wipe out sin; and every human being has a right to achiev moksha provided he finds shelter in God.
CAUSES OF POPULARITY
Significantly the cult of Vasudeva-Krishna spread rapidly and became most popular. Reason being, Krishna, satisfied all human urges and emotions as son (of Devaki and Yashoda as conwerd (protecting cows much venerated both by the Aryans and the non-Aryanas), as human or divine lover (of gopis and young women), as saviour (of the oppressed by killing his maternal uncle Kansa and demons, of the non-Aryans by hcallenging the Vedic God Indra, of the women by saving Draupadi's honour) as ambassador (playing this role for the Pandavas), as charioteer (by driving Ajruya's rath) as preacher (upadesha in Kurukshetra), as dvarapala (gatekeeper and receptionist when the Pandavas conducted the Yajna) as the lover of tradition (his presence at the time of the conduct of sacrifices) as friend (of the poor, the cowherds and Sudama), as lover of music (playing flute), as just kind (of Dwarka) and by his emphasis on a combined pursuit of Inana, karma and bhakti made brahminical religion more flexible and more easily acceptable. It was this tradition which cound and did attract foreign mlecchas and which transformed Brahmanism into Hinduism.
To sum up, Krishna probably even more popular than Rama, is a divinity of a rare completeness and catholicity, meeting almost every human need. As the divine child he satisfies the war maternal drives of Indian womehood. As the divine lover, he provides romantic wish-fulfillment in a society still little tightly controlled by ancient norms of sexual relations. As charioteer of the hero Arjuna on the battle filed of Kurkshetra, he is the helper of all those who turn to him and even saving the sinner from evil of rebirths if the has sufficient faith in the Lord.