The straight answer to this question is: because it is real. The more sophisticated one will encompass multiple reasons. We always learn that life and even religion is not that simple:
1- The reality of Islam: It is very reasonable attitude for humans to be suspicious and critical. Islam to a lot of Arabs and even non-Arabs is like a trade. Do you really want to pay to buy certain goods. Do you really want to spend a lot of time, effort and endurance for a true or false deal. Thus most people even who inherited the religion runs it well in their mind: Is it real or not.
2- Fulfillment: the fulfillment in Islam is related to the spirituality of the religion and the materialistic fulfillment. You can easily have it both ways.
3- The integration of life: Islam not only cause Muslims to live as better individuals and more holly but integrate them as a loving community.
4- Living along non-Muslims: Like a good Jew when he will be in any community he will be looked at as the champ of equality, justice and human rights. Good Muslims are fare people who would require equality and justice for all.
5- Honesty and transparency: unlike the Arab leaders now who plays politics in the dark. They promise their people justice and democracy and does every thing in the dark to get more control and greed over their people rights and wealth.
6- Clean and pure life: people whatever their background if they would to live a pure and clean life they would feel rested, calm and happy. You give up a little materialism for a lot of spiritual gain and peaceful life.
7- Islam believes in social classes: Though the best of us for God is the best in fear to God, Islam accepted the reality that we can not be all equal. For example in marriage Islam prefer you to marry from your same socio-economic level as an individual. But as a group Islam help us to tighten the gap between the rich and poor and teach us that best of us who has the best manners particularly with people who are weaker or less fortunate than him. For non-Muslims Islam did not break the rich or more affluent or the people. Even when they turn into Islam. The religion acknowledges the presence of classes. But it makes people look more beyond the classes. There is no social war in Islam.
8- God forgiveness for sinners: as in other major religions sins can drag man to hell and meanwhile no one can be so desperate that God would never forgive his sins whatever great they are. Thus religion particularly Islam has always an open door to sinners. Meanwhile it is against organized sinning. This is a community that all indulge into sins. Organized sin or overwhelming sin equates to God organized wrath and overwhelming wrath.
9- Religion comes before personalities: For Muhammad (PBUH) the worst of hypocrites and unbelievers who were against him he never had a personal revenge against any of them. A lot of them became very good Muslims. They simply found it is something very different and it is not about the victory of Islam but about getting God word to all people and liberate all people from all kinds of tyranny. If you find tyrants or evil leaders in Islamic history it does mean that humanity has its evil even from any religion.
10- The leaders of Muslims as models: in early Islam were more pious and devotee to their religion. People looked at them as new breed of people. Most people looked seriously and studied seriously their religion. It is like the Gospel or good news of Jesus. There is God he connected again with another human (Muhammad) through his angel Gabriel. He is not a liar and not crazy. It is an astonishing story but more believable than an elephant that was an ant or the thousands of species that came from one atom or single cell.
11- Removing all forms of oppression: there is no compulsion in religion. Simply God wants to go to him with your complete mental and spiritual will, There is no middle man in Islam you respect Imams but all your relation with God is one in one. There is no oppression of women, workers, minorities, foreigners.... Islam means that you submit to God but in the same time you liberate your brain and soul. You do not become under the oppression of sins, people, governments, religious institutions,... It means you live with freedom and dignity more a life of worshiping, awareness and wisdom.