Because Allah the Exalted has said (in Surah An-Nisa) «يا أهل الكتاب لا تغلوا في دينكم ولا تقولوا على الله إلا الحق» "O People of the Book! Do not exceed the limits in your religion."

Narrated Abu Huraira: The Prophet said (to his companions), "Do not fast Al-Wisal." They said, "But you fast Al-Wisail." He said, "I am not like you, for at night my Lord feeds me and makes me drink." But the people did not give up Al-Wisal, so the Prophet fasted Al-Wisal with them for two days or two nights, and then they saw the crescent whereupon the Prophet said, "If the crescent had delayed, I would have continued fasting (because of you)," as if he wanted to vanquish them completely (because they had refused to give up Al Wisal).