Because Zionism is an idea of establishing a Jewish homeland in a place where 90% of native population was not Jewish. That is a terrible mistake for any rational person on earth and not only Muslims. Here are some useful links for you to get the full picture my friend:
http://en.wikipedia.org/wiki/Mandate_Palestine#Population
http://lawrenceofcyberia.blogs.com/news/2010/03/palestinians-made-the-desert-bloom.html
http://domino.un.org/unispal.nsf/90634f6f0dc8cd1b85256d0a00549202/a73996728ba8b94785256d560060cd1a?OpenDocument
http://www.rondavid.net/King-Crane-commission.htm
As you can see, Israel is a terrible mistake. Also, check out this interesting article: Zionism - A Terrible Disease of the Mind
http://www.palestineremembered.com/Articles/General/Story12190.html
The problem is not only in the creation of the Jewish state on Arab land, but also that Israel after more than 62 years of its establishment failed to become civilized and democratic state. Check this out for example:
http://www.uruknet.info/?p=m71434&hd=&size=1&l=e
http://www.hrw.org/node/95113
http://www.youtube.com/watch?v=EyVPSU1xtPM
http://stopthewall.org/FAQs/33.shtml
http://www.ifamericansknew.org/
And not only Muslims are anti Zionists, there are many Jews who oppose Zionism and call for exchanging Israel to democratic state where both Jews and non Jews have equal rights, and where immigration and settlements are not based on religion. Check out jewsnotzionists.org for more info
And my friend, this is the 21st century, you can't steal other peoples land in the name of God anymore. And if God really exists i don't think that terrible racism would be "his will".