One of the most important things in islam is unity. It has been directly emphasized in islamic teaching and islam has systematically inbuilt factors that incourages muslims and make them united unlike chritianity and other religions. Just to give you an example, there is a famous hadith (the prophets teaching) “The parable of the believers in their affection, mercy, and compassion for each other is that of a body. When any limb aches, the whole body reacts with sleeplessness and fever.” sahih albukhari 6011 And another one "A Muslim is a brother of another Muslim, so he should not oppress him, nor should he hand him over to an oppressor. Whoever fulfilled the needs of his brother, Allah will fulfill his needs; whoever brought his (Muslim) brother out of a discomfort, Allah will bring him out of the discomforts of the Day of Resurrection, and whoever screened a Muslim, Allah will screen him on the Day of Resurrection ." This was on the prophets last (dying) sermon As a muslim it's compulsory to attend prayers 5 times a day in a local mosque with the congregation, 1 time a week in a bigger congregation, 2 times a year in an even congregation and then there is a worldwide islamic pilgrimage that further unites muslims from every corner of the world. This ingrains sense of unity into every muslims mind. I dont think there has been or is ever going to be anything that can get chritianity or any other religion as united as muslims. Moreover muslims all over the world have the same version of the same scripture. The right question is not why Christians are not united but why muslims are united.