Because if they start doing it, they'll get more requests to do it in the future. And then they'd have to make more process around it and pay people to execute that process, and then they're in the business of selling small bits of criminal evidence to random cranks.
There is probably not an infinite supply of dna evidence, and testing it is destructive. So I can imagine they don’t want to testany plausible theory but save it for when they are really convinced.
