So then Microsoft is responsible for leaving it out.
Not only MS. The hardware supports Verizon's bands, but they are disabled via software. Why? There is no official comment from either MS or Verizon, so we don't know.
However, no capitalist company limits the number of customers they can sell a device to without reason, meaning MS was coerced by something or someone. We just don't know by what.
Maybe AT&T demanded the device be a carrier exclusive?
Maybe MS felt that the ability to update Verizon L950/XL devices directly was a non negotiable requirement, to which Verizon replied: "no, and if you try we'll make life miserable for you and your Lumia customers"?
Maybe it was something else? We don't know.
All we can say for sure, is that MS expected the problems caused, by allowing the phone on Verizon's network, to outweigh the expected benefits.