I hear what you both are saying, but I don't believe the lack of an arrest at this point means they don't have enough evidence for probable cause. While Ms. Turner would almost certainly benefit politically from an arrest - there still is time before the election for that to happen - and what better boost to an election than an arrest the Friday before? However, it's also been pointed out that Sheriff John hasn't publicly endorsed her (the only one of the four sheriffs in her jurisdiction to NOT do so), so it's possible that has an impact on the timing as well.
It also could be as simple as they are waiting for hunting season, hoping that they get lucky re: finding SM's remains. JMO, I'm personally of the mindset that they have a lot of evidence (based on the fact we're aware of only a little bit of what they have, and that alone, IMO, is extremely compelling), and are simply taking their time to make sure they get it right. MOO.