People's religion is very important to them. It is a guiding principle to their everyday lives. When someone decides to defame their religion by making a cartoon version of someone who is the biggest prophet ever, you're undermining everything that religion stands for and making yourself look prejudiced and dumb.
The cartoons are clearly racist towards Muslims. I think they are even intended to insult Muslims. Although I believe in freedom of speech, the cartoonists clearly made these cartoons just to stir up a reaction. Imagine if the same thing happened in publications of Jesus? Christians would find it extremely offensive.