What do they benefit from making an early arrest? If they do not have enough evidence to prove to a jury that a murder was committed without the addition of a body then he could well be acquitted.
I don't believe LE named him a suspect in hopes he would make a mistake. I tend to think they know he was involved in a probable murder and are simply waiting for a more likely conviction with a body. (I am so sorry, Sparky

)
Scott Peterson was not arrested until Laci was located. Doug Stewart was however convicted without Venus being recovered. Josh Powell has yet to be arrested and Susan remains missing to this day.
The mere fact that LE has not arrested Dale for murder does not satisfy my mind that no murder was committed nor relinquish his, imo, probable involvement.