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 to where you were before the change. History means a lot IMO & by changing the URL the site history is gone so basically you're starting over from the beginning in some ways.
Since RPM is just a calculation from your earnings that are driven by the CPC when your CPC dropped drastically the RPM had to drop with it.
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 to where you were before the change. History means a lot IMO & by changing the URL the site history is gone so basically you're starting over from the beginning in some ways.
Since RPM is just a calculation from your earnings that are driven by the CPC when your CPC dropped drastically the RPM had to drop with it.
