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.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / Q5141