The time that has passed is definitely a factor but this may be what the DA's trump card is...
now I hear craig silverman saying that while the boulder police were exchanging emails with him (he thought it was Patsy) - he drops a hint that Karr may also have mailed them a package of some kind, and that package had his DNA on it (in some form) along with who knows what else. and this is the DNA they used to match to the underwear.
did anyone else see Silverman on OReiily?