Because in Islam they are the SAME god!
if you study islam you will see that Jesus was one of gods prophets, just like the prophet muhammed and every other prophet before him. They are all the same; moses, jesus, muhammed. Moses brought judaism, Jesus brought Christianity. All prophets are regarded in the same esteem in islam; none are better than the other or more sacred, all prophets are the same and equal.
To put it in simple and layman terms; god sent all the prophets before prophet muhammed to spread the word of god...they all tried to spread the correct word. Unfortunately, each time a prophet was sent the word of god and religion deviated from what god intended. So god sent more prophets, until muhammed, who spread the correct word of religion / god. So muhammed was gods last messenger and therefore the word he spread was the correct version. That is as simple as i can make it. In islam we are taught to respect all religions; christianity, judaism, etc...that is because really Islam contains parts of all of them, and why there are so many similarities between them; because they came before islam. So any reference to god means it is the same god, as it was the same god who in essence created all these religions. So when people say Allah as being anyone different, its not because it is the same. Allah is just arabic for god, nothing else, not some super being different to any other god worshiped in religion.