That's one of the things I sort of admire McConnell for - he is a black widow spider that hangs in the corner waiting for his political adversaries to make a mistake - then he pounces. Is he lacking in some spine and conservative fighting spirit? Fully, but people (especially Senate leaders) are a package deal. They are often a mixed bag - especially McConnell's tendency to retreat from political confrontations. 

Maybe he is tired of trying to herd the cats that make up the Senate GOP.