Because Islam (meaning: submission to the will of God) is the true religion passed down from Adam to all the previous Messengers: Noah, Abraham, Moses, Jesus and Muhammad.
Their ultimate message was to tell their people to believe and worship only one GOD and that is what the Muslims (meaning: one who submits to God) are still doing today (5 prayers per day, 1 month of fasting, charity, pilgrimage, etc).
Firstly, we must all understand that the reason of our existence on earth is to worship God and this life is a test to see if you will follow this commandment (which is well explain in Islam and the Quran) or you will follow your desires (like animals). The final decision is yours, Worship God (by the way of the true religion Islam) and you will be rewarded with paradise for eternity or neglect God and follow simply your desires and you will be in Hell Fire for eternity.
Please, just read the Quran once, and by God's will, you will see the truth in those final words that God revealed to us.
Answered Feb 15, 2012
Edited Feb 15, 2012