Why is a vow not encouraged in the same way that supplication is, even though a vow is "a prayer, a thanksgiving, and a promise to God" and is considered "a prayer and an entreaty to God" expressing gratitude and praise?

Supplication (Du'a) differs from vows (Nadhar) in that supplication is submission, humility, and a plea to Allah. It is one of the most prominent forms of worship in times of ease and hardship, and it is praiseworthy and desirable in Islamic law. The Prophet, peace and blessings be upon him, described it as worship, and Allah answers it, or saves its reward, or averts evil through it. As for vows, their apparent nature is one of exchange and transaction. The Prophet, peace and blessings be upon him, forbade them because they do not repel anything from destiny and are extracted from the miserly. Among the reasons for their prohibition is the belief of the ignorant that they can repel destiny, or because they involve an act of closeness (to Allah) through a transactional or forced manner.