For some reason, I think it's because of the new style "set" and "get" methods, you can no longer set the selected property on an enyo.Select at create time. The only way to set the selection is after render.
Here's some fiddles:
enyo 2.2.0 http://jsfiddle.net/mmwfq7fe/ — works
enyo 2.4.0 http://jsfiddle.net/mmwfq7fe/1/ — works
enyo 2.5.1 http://jsfiddle.net/mmwfq7fe/2/ — broken
enyo nightly http://jsfiddle.net/mmwfq7fe/3/ — broken
update: This is in our queue to review the PR linked in the forums in the next week or two.