Because they contended against Allah and His Messenger

Remember He covered you with a sort of drowsiness, to give you calm as from Himself, and he caused rain to descend on you from heaven, to clean you therewith, to remove from you the stain of Satan, to strengthen your hearts, and to plant your feet firmly therewith. {11} Remember thy Lord inspired the angels (with the message): "I am with you: give firmness to the Believers: I will instil terror into the hearts of the Unbelievers: smite ye above their necks and smite all their finger-tips off them." {12} This because they contended against Allah and His Messenger: If any contend against Allah and His Messenger, Allah is strict in punishment. {13} Thus (will it be said): "Taste ye then of the (punishment): for those who resist Allah, is the penalty of the Fire." {14} O ye who believe! when ye meet the Unbelievers in hostile array, never turn your backs to them. {15} If any do turn his back to them on such a day - unless it be in a stratagem of war, or to retreat to a troop (of his own)- he draws on himself the wrath of Allah, and his abode is Hell,- an evil refuge (indeed)! {16} It is not ye who slew them; it was Allah: when thou threwest (a handful of dust), it was not thy act, but Allah's: in order that He might test the Believers by a gracious trial from Himself: for Allah is He Who heareth and knoweth (all things). {17} That, and also because Allah is He Who makes feeble the plans and stratagem of the Unbelievers. {18} (O Unbelievers!) if ye prayed for victory and judgment, now hath the judgment come to you: if ye desist (from wrong), it will be best for you: if ye return (to the attack), so shall We. Not the least good will your forces be to you even if they were multiplied: for verily Allah is with those who believe! {19}

(Surah Al-Anfal)