Generally two reason, changes in the Soviet Union after the death of Stalin and great power rivalry.

Mao aligned very much with ideas and methods of Stalin, the cult of personality, heavy industrialisation, urbanisation and the ideas of the revolution and global communism. After Stalin's death however Khrushchev began to dismantle Stalin's cult of personality, give greater importance to the production of consumer goods and move away from the idea of inevitable war with the capitalist world instead seeking for Peaceful Coexistence. All of this proceeded to annoy the Chinese to no end as they saw it as betraying the revolution.

Finally there was the issue of great power rivalry in the communist world. Russia viewed itself as the centre as it was the oldest and most economically powerful (and it had direct control over eastern Europe). China on the other hand claim it would take the leadership of the Communist as its model of peasant revolution would be more successful in Asia than Russian urban revolution.