If you go through the stories of people who converted or reverted to Islam you will find a lot of reasons. It is quite likely for one person it would be one single reason but for most multiple reasons:
1- Try to find the plain truth of God.
2- Unsatisfied by the religion person have.
3- Converts from Christianity: The dispute between Churches about Who is really God, Mary and Jesus. This divinity relation was never told by Jesus and have been speculative for centuries. The three main pillars of religion God, commandments and salvation are still being argued after 2000 years.
4- Accepting the reality of Jesus in Islam and our salvation by his teachings not his blood.
5- The high authority of God in Quran and meantime the close proximity to each of us. No middle man no one to confess to or intercede to. It is one on one perfect relation 24 hours a day seven days a week.
6- The great equality in Islam that put all people in front of God in the same rows praying and kneeling to him.
7- The personal freedom that a Muslim enjoy particularly from his ego and evil inside that self destroy him.
8- The closeness of good Muslims and the way they treat each other.
9- Try to escape materialism and have spiritual life.
11- Knowing good Muslims with good morals attract non-Muslims to read about Islam. Interestingly Muhammad (PBUH) has a saying "my miracle after my death is a good Muslim". Since a good Muslim will invite people to read the miraculous Quran.
12- Marriage is a factor to read about Islam but no compulsion a Muslim husband can not and should not force his non-Muslim wife to be a Muslim.
13- The thinking of after life particularly more with senior people thinking of eternity.
14- Outreach of Muslims to convert non-Muslims at personal level and in media avenues like my blog.
15- Believe in Quran by trying to dispute it. This is more common in highly logical and sophisticated people.
16- People who are into comparative religions.
17- After dialog between major religions clergy.
18- Simplicity to become a Muslim.
19- Islam invites believe as you read from the verses of Quran.
20- Quran tempts people according to their goodness so people who are just and try to find God Quran talk to them about reasoning and he beauty of creation. People who are hard, evil and have false pride Quran intimidate them with hell fire particularly when persecuted the Muslims.
Thus reasons vary but all meet in one way true religion with true straight path. The amazing thing Islam accepts even skeptics. Quran is a dialog between God and you. He reads your mind and skepticism and give you the answers all along as you read it. Quran gives you the reasons why there is God and how with prayers and supplication you stay close to him. Satan in the process try to get you astray by denying God or partnering him with others or he gets you in an evil to do. He is always busy try to get you from somewhere.