There could be a few reasons but one could be that there were some higher paying advertisers that were placement targeting your site & by changing your URL you lost all of them until they find your new domain. Another is that even though you're using the same ad code since the ads are on a different site the bots aren't placing the same ads to the new domain that they were placing on the old.
Over time as the bots crawl your site more & advertisers find the new URL it may improve but it may take a while & it may not ever get back...