Thanks, that really helped isolate the problem.
I confirmed that all of the values were under the Model attribute, but the Sharepoint View had an item limit of 100 per page.
For some reason, Sharepoint is only sending data from the first page.
As a work-around, I increased the number of items on the Sharepoint View higher than the number of items on the list (1000 items), and now it is correctly populating the drop-down list.
Since this will likely come up that I will need to design a form where this work-around is not acceptable, is there a better solution?