Good question.
By now, investigators must have a ton of evidence.
Besides the physical evidence we saw them removing from the home (how many times did the CSI folks go back, three times?), but the digital evidence from surveillance cams, phones (Gannon's at least, but I would suspect the step-mom's as well), and any other evidence that we haven't heard about yet (there's always evidence we never knew about, it comes out at the trial and we all go WHOA!)
So the answer might be, that's a LOT of evidence to go through and paint a clear picture to obtain an arrest warrant after the probable cause has been signed off on.
Just my guess, but they're building a solid case - and still looking for Gannon at the same time.
That's a lot of work.
God bless them in their efforts to bring justice to Gannon and his family.