Because of the saying of the Prophet (peace and blessings be upon him): "The people should be led in prayer by the one who is the best reciter of the Book of Allah (the Holy Qur'an)."
And a disbeliever's performing the prayer does not become proof of his being a Muslim unless he verbally declares Islam before the prayer.
Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "I have been commanded to fight the people until they testify that there is no god except Allah and that Muhammad (peace and blessings be upon him) is the Messenger of Allah, establish the prayer and pay the zakah. When they do these things, they have protected their blood and their wealth from me, and their reckoning is with Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5141