Sometimes people do not want to change their belief in a theory because:
* they just want to continue to believe their current understanding
* they do not know about new evidence
* they reject the obvious evidence
* all of the above

One of my passions is keeping up with creationist (and I include Intelligent Design here) attempts to  change what science is.    They wish to supplant the reason and evidence that is the basis for good science with faith instead – specifically their faith. 

Sometimes, actually often, it can become discouraging looking at how many battles must be fought to ensure our schools continue to teach good science; listening to all the politicians expressing their ignorance of science by expressing doubts about evolution; and seeing all the letters and forum responses from those who let their religion totally blind their ability to reason and fairly judge evidence. 

For example, consider these quotes from various prominent politicians:

“There are clear indications from our people who have amazing intellectual capability that this didn’t happen by accident and a creator put this in place,”

“Now, what was his time frame and how did he create the earth that we know? I’m not going to tell you that I’ve got the answers to that,” Perry said. “I believe that we were created by this all-powerful supreme being and how we got to today versus what we look like thousands of years ago, I think there’s enough holes in the theory of evolution to, you know, say there are some holes in that theory.”  Governor and Presidential candidate Rick Perry

explanation:

the sequence of amino acids in a protein determines its function. the dna sequence of a gene determines the amino acid sequence of the resulting protein. thus, any changes in dna sequence can result in changes in protein function. this is called a mutation.