i'm glad you know that now.
, islam and christianity are like one religion but with some differences.
well the easiest answer i can give you is because the Kur'an (the word of Allah) says so.
one of the reasons why i think of that is because Muhammed (saws) was a prophet sent to humanity, unlike all the other prophets who were sent to one clan or one tribe. the bible and torah fortolled that a man named Ahmed was going to be the last prophet (since you are a christian you probably don't like me saying this but i will have to say it, i hope you can forgive me, don't take it as an offence or dissrespect agaisn't christianity, the christians and the jews both changed the verses of the holy books that spoke of Muhammed (saws), the jews also changed that which spoke of Jesus (as) and it cannot be found in there about Muhammed (saws) as the last prophet) there are other reasons but thats the one that i have chosen to tell you about, others are that, the kur'an says so (just as i said before), the small signs of judgment dy that we muslims are tolloed of in the kur'an are starting to appear,
there is none other because like i said, Muhammed (saws) is a prophet sent to humanity, his message (that of Allah revealed to Muhammed (saws)) will never dissapear untill judgment day.there will always be muslims in this world. unlike with other prophets whose people eventually failed to follow the message sent to them by their prophets. when ever the message dies (when there are no more beleivers) then Allah sends a new prophet, but now we have been given a promise by Allah that islam will live untill the final day (the day of judgment), and as mentioned before some of the small signs of judgment day have already appeared ie. people will love dogs like they love their children, men will look like women and women will look like men (homosexuals), etc....
i hope that information is enough for you, if not please let me know or make another question if you wish.