What is the interpretation of Uthman's saying to Ali, "But we were afraid," in response to his question about Mut'ah?

Ali and Uthman, may Allah be pleased with them both, differed concerning mut'ah (pilgrim's enjoyment), which here refers to mut'at al-hajj (Hajj with mut'ah). Uthman, may Allah be pleased with him, forbade it, while Ali, may Allah be pleased with him, enjoined it. Then Ali said to Uthman: "You surely know that we performed mut'ah with the Messenger of Allah, peace be upon him!" Uthman replied: "Yes, but we were afraid."

Uthman, may Allah be pleased with him, did not clarify the reason for their fear. It was said that Uthman's statement "we were afraid" referred to the Day of Umrat al-Qada' (the Compensatory Umrah), but that year was not truly tamattu' (Hajj with mut'ah), rather it was Umrah alone. It was also said they were afraid that the reward for one who performed ifrad (Hajj alone) would be greater than the reward for one who performed tamattu', but this is unlikely. The more probable explanation is that Uthman was referring to the Prophet, peace be upon him, changing Hajj to Umrah during the Farewell Hajj to dispel the Quraysh's belief that performing Umrah during the months of Hajj was forbidden. This occurred at Hudaybiyah, where they were afraid of fighting breaking out between the Muslims and the polytheists.