Because that would involve two full network test and releases instead of one. Verizon is just not going to do that and, tbh, I don't blame them. Their testing process is too rigorous and time consuming as it is.
And, to attempt to look at it from their point of view, they will select the update that lines up more with their initiatives, which is Update 1 with VoLTE. When that network upgrade fully launches and the marketing department starts to push it (like with XLTE), they want as many devices as possible to have it and that would require all of their WP's to have Update 1.
And, for the record, I'm not trying to excuse how long it's taking to get this update. I'm just trying to find plausible reasons why it would be taking this long and this one makes a lot of sense to me.