(I'm not making an argument against evolution. I happen to view it as fact)
I think there are three reasons for this phenomenon.
First, the method at which it was arrived is not, strictly speaking, the scientific method. In the scientific method, an hypothesis is formulated, then a test of that hypothesis is constructed to observe the variable in question, data is collected, and it is proven or disproven.
Evolutionary biology works in the opposite direction. In evolutionary biology, data is collected and compiled, and an hypothesis is formulated afterwards. If subsequent data disprove this explanation, the explanation is revised, but there's no way to devise a controlled test, because you can't hold any of the environmental variables constant. For this reason, evolutionary biology would probably better be described as a "logic system" than a "science". This doesn't mean it's not true. It just means it isn't truly scientific. The same could be said for certain aspects of astrophysics, but enough is actually scientific that we can forgive them that.
Second, while several examples of microevolution have been found, virtually no examples of macroevolution have been observed in human history. Granted, we haven't been observing for very long, but that's not really an excuse. Here's why: We believe humans evolved from lower primates over the course of a period of time on the order of millions of years. There are bacteria with a generational time that is about 1/350400 of ours. After being aware of these bacteria for several decades, shouldn't we have seen an evolutionary innovation at least as significant as the primate/human transition by now?
Third, and this is the least-reasonable reason, but probably the most important one in terms of creating evolution deniers. The most publicly prominent evolutionary biologists in the world are militant atheist dickholes. I don't know of a single scientific discovery Richard Dawkins has made, but I know of a ton of books he's written about what a bunch of silly bitches the faithful are. If he's a dedicated scientist, why does he expend so much energy on philosophy?
People are spiteful, and the best way to get them to refuse to listen to what you have to say is to spit in their face. No other scientific discipline feels the need to go on the attack this way. I think it's because every other science is a little more comfortable with their status as a "science" rather than a "logic system", or should I say a "belief system".