That is because onload is a document(body) event.
demo: http://jsbin.com/benosofo/1/
The onload event can only be used on the document(body) itself, frames, images, and scripts. In other words, it can be attached to only body and/or each external resource. The div is not an external resource and it's loaded as part of the body, so the onload event doesn't apply there.
from: https://stackoverflow.com/a/4057251/2213706
