I tried to find a clear explanation a couple of times from someone I know who speaks both Arabic and English...I don't think he had a clear answer. I tried looking up the linguistic complexities of Arabic and English....and honestly, English looks more complicated. I guess Arabic is highly contextual but...so is English. You can change the meaning of English sentences just by emphasis on a different word. English has a tense that only 3 or 4 languages have. I've got a lot more sympathy for English as second language speakers since I learned how difficult it really is.
Arabic, for whatever reason, hasn't changed much in hundreds of years.
I've come to think it's the lack of apologetics. Christianity got big through conversion, then legal mandate, then conquest and localization. Islam was spread through violence...and oppression of non-muslims. It prevents widespread denominational changes with punishment of death. Apostates are also killed. This means there's rarely any time or place before the last couple hundred years where Islam needed any defense of its faith...of its beliefs...so that history of apologetics that exists in Christianity doesn't seem to be very old in Islam and I suspect most adherents rarely have their beliefs challenged.
I don't typically like oversimplification of large worldview type belief systems...but I'd say the big contrast between Christianity and Islam is found in the moral character of its founders. Jesus comes upon the stoning of an adultress and says "He that is without sin among you, let him cast the first stone at her". He exemplifies self sacrifice as the highest good.
Muhammad comes upon an adultress who seeks punishment...and commands she be stoned to death.
In both cases, we see what adherence to the faith means in each religion. One is about forgiveness, humility, and self sacrifice. The other is about strict obedience to the word of god.