McConnell is winning the primary because he is in essence a Tea Party candidate. Look at his voting record. He even supports a federal "right-to-work" law (which nobody except the Tea Party likes).
Not uncommon. Tea Party challengers (or the everpresent threat of one) over the past few years have moved most Republicans lawmakers notably rightwards in their voting records. IMHO, there are substantially fewer RINOs in the GOP than TPIABNs (Tea Partiers In All But Name).
Admittedly, I don't think that acronym will catch on as it doesn't exactly roll off the tongue like "RINO" does.