Because, unlike Intelligent Design, the theory of evolution is a scientific theory based on conclusions drawn from empirical evidence. It is not a religious viewpoint. It asserts no religious claims, and it does not prefer one religious view over another.
We are looking at this two different ways. ID, from the little bit I know about it, says God must have facilitated conditions for the world to be formed and life to evolve. Its not challenging evolution or the big bang, just saying that there was an underlying factor and that millions of random events and blind chance couldnt allow for it to be done. Then evidence is presented about the rate of the universe expanding, amount of gases in universe, statisitical improbability, etc.
Now, evolution has evidence like adaptation seen in viruses every year, bacteria strains adapting (this is how evolution can be remotely brought up in class without talking about Darwinism. Whoever tried to make me look like a fool earlier, nice attempt), and of course fossil evidence. They even have found another "missing link" which was previously the best argument against evolution.
So, evolution has all this evidence, but there is little to none for the actual creation of life. There was a handful of experiments where adenine and Thymine (IIRC) where produced in a lab, but thats hardly evidence. Thats like finding flour and an egg near each other and calling it a loaf of bread.
Some interesting scientific research has been performed into estimating the minimal gene set required for a living cell, and this has been followed up by experiments on Mycoplasma genitalium, the simplest known cell, to see how many genes could be damaged before the organism failed to survive and reproduce. The original estimate determined that approximately 256 genes would be the minimal set required to support a viable cell. The experiments on Mycoplasma genitalium, however, never managed to reach this low level: their experiments suggest that between 265 and 350 genes are necessary.
Even if we take the lower estimate of 256 (which isn't too far below the lower end of the experimental data), then it's clear that we have a problem that's far too big to produce by chance. Genes can be made up of 100s of pairs of nucleotides, so I am not impressed by some adenine being made. How about making a living organism from nothing before claiming you have empiracle evidence that me, you, Randi, and every living cell in in this world came from essentially nothing.
In summation, ID simply says our world could not have randomly come from nothing without the help of a God, fairy, hobbit, or magic. It does not dispute evolution at all. until science actually has empiricle evidence that life can arise from nothing, let only absorb fully broken down inorganic molecules as energy, teaching ID or any other theory has no more evidence on the start of life than evolution. Because God/Magic/Hobbit can never be proven, the burden of proof is on you to show that a living organism can arise from non-living material. Good luck.
http://www.cbs.dtu.dk/staff/dave/min_gen_ref.html
