On my own understanding, maybe because other people are not aware of islam.They know only those things which so-called media tell them.No one tries to read Quran and tries to know what islam is all about.
For understanding islam don't look to muslims,look to Quran.Because you can't paint all muslims with same brush i.e terrorism and extremist.
Every muslim is different from other.Not all muslims are exemplary muslims.There are some fundamentalist muslims who are really good but there are also some muslims who are not good at all.There are muslims who have a good character but there are also who have a bad characters. Before we Judge an individual we must first understand the real real of being his/her personality.