In a single-party system, you can't allow dissent or you will end up with multiple parties, if enough of the dissenters agree with each other.
When someone speaks against the government in China, particularly someone with a high profile like Jack Ma, the government is worried that other people might agree with him, and they might form a movement that demands change and challenges the party's policies.
If the party allows this to happen, that would mean they recognize the legitimacy of political action originating outside of the party. That would be an existential threat to the single-party system.
(Also, the last time the party allowed public dissent, it grew out of control and turned into the Tiananmen Square protests. The party's current leaders remember that time well, and the lesson they took from it is that allowing dissent was a serious miscalculation.)