In 1925, John Scopes, a high school biology teacher, was put on trial in Tennessee for having the audacity to teach evolution to his students. In the 21st century, teachers don't have to worry about being arrested for teaching this fundamental topic in science, and the Supreme Court declared teaching creationism unconstitutional in 1987, but that hasn't stopped state legislators around
1 ) Evolution is the basis for all biology. Without it, much of biology and modern medicine just doesn't make sense. There's general agreement that good science education is needed to produce a populace capable of handling our increasingly technological future. Evolution has to be part of that, but sadly, it rarely is. A recent poll of high school biology teachers found that only 28 percent consistently teach evolution.
2 ) Teaching unscientific "alternatives" only confuses students. "There is virtually no scientific controversy among the overwhelming majority of researchers on the core facts of...evolution," Alan Leshner, executive publisher of Science, wrote recently to two Tennessee legislators. "Asserting that there are significant scientific controversies about the overall nature of when there are none will only confuse students, not enlighten them."
3 ) Science-based industries might conclude the state is anti-science. Florida is considering its own law to require "critical analysis" of evolution, which could open the door to unscientific theories being presented in the classroom. In response to the measure, the Florida Academy of Sciences issued a statement noting that the measure would "undermine the reputation of our state and adversely affect our economic future as we try to attract new high tech and biomedical jobs to Florida."
4 ) Anti-evolution theories aren't science and don't belong in a science classroom. Whether you call it creationism, creation science or intelligent design, it isn't science and shouldn't be taught alongside scientific theories. I could see the story of creation being taught in a history class, while studying the creation mythologies of various world cultures, but anything else is promoting religion and is unconstitutional in a public school.
5 ) If it goes to court, the anti-evolution side will lose, potentially costing a school district or state a lot of money. Case in point: Dover, Pennsylvania. The Dover Area School District was sued by parents after it mandated the teaching of intelligent design. The district lost, spectacularly (pdf), and paid more than $1 million in legal fees. Defending the teaching of anti-evolution theories now could potentially cost millions more.