Alma’s people were not in the usual position of a rebellious people chastened by afflictions until they repent. They were already righteous and surely prayed daily, so why does Mormon say they “began to cry mightily to God.”
The original tribute status was accepted and acceptable to Alma’s people. It was when Amulon increased the burdens upon them that they “began to cry.” It is this increased persecution that has pushed them to the limits of their abilities. What “began” was not normal prayers, but specifically prayers for deliverance from what had become an intolerable situation. Of course, in this increased persecution they would increasingly turn to Yahweh for relief, as they could find none in their own powers.