Let's think this through. The possible reasons that Verizon doesn't have more than one Windows Phone could be: because it is CDMA, because Microsoft hasn't pressured them enough, because the OEMs aren't providing them with any choices, or Verizon doesn't want to carry a WP selection. I honestly believe all of these are correct in some degree.
Let's start with the fact that it is a CDMA carrier. It may appear that CDMA carriers have a lot less to choose from, but it seems that Verizon is doing quite well when it comes to having the latest and greatest Droid selection. They have the tons of Motodroids (including the bionic), tons of HTCs (incredible 2, thunderbolt, etc), and more.
That shifts the burden of blame to the OEMs, Verizon, or Microsoft. Microsoft, like others said, is not the company that makes or sells the phones. They have provided their OS to support CDMA and now they are more or less done. That leaves the OEMs and Verizon. Verizon claims that it only puts out what OEMs give it, while Verizon was the last carrier (after even Sprint, which ideally would be less desirable to OEMs, as they aren't nearly as big as Verizon) to receive a Windows Phone. So where does the fault lie?
I believe right now the fault is in the OEMs. I feel like it is too coincidental that neither Sprint or Verizon have an announced WP. They probably aren't creating phones for Verizon and Sprint yet, though, because they are still CDMA, and so they might have put them on the back burner (although I can't imagine why anyone would neglect Verizon Wireless). However, I feel that both Verizon and Microsoft are to blame, as well. Microsoft definitely has the weight to make OEMs produce their devices, so they obviously don't believe that their Verizon and Sprint market shares are either in trouble or big enough to worry about. As for Verizon, it has been clear that they've had some hostility toward WP in the past (although, they seem like they're slowly coming around).
tl;dr - Hierarchy of fault: OEMs > CDMA > Verizon > Microsoft