Casey has been arrested, for other crimes. She's in jail and not getting out. There is no rush to hastily charge her for more crimes at the moment. It's better to have all it i's dotted and t's crossed than to rush into a murder charge. Which BTW, a good DA would not allow. And I would bet that if there is a possibility that this is a death penalty case, it will go before a grand jury before and indictment is handed down. That's standard operating procedure these days. A grand jury can take 6 months or longer. There are cases that I know of where the grand jury has been seated for years.
DNA tests are not a quick thing. I seriously don't understand where people get that idea. You have prelim results, then more testing on those results, than more testing on those. Mark F's comment the other day about testing in this case was very enlightening. IIRC, He said if the prelims in this case came back with nothing, then the state wouldn't bother doing the more in-depth expensive tests that is now being conducted.