Why was religion not allowed in Communist China?

Communist governments traditionally do not approve of religious observance; it encourages people to gather in groups and possibly exchange ideas which are not conducive to obedience under a communist regime. For the same reason, religious organisations discourage communism, and the more fundamentalist religions discourage education outside of the church while the seriously restrictive religions discourage contact with those outside their particular church. There isn't much difference between hard-line communism and fundamentalist religious groups. It's all about control, and ultimately, power.