While Rapidshare AG says they are moving to Rapidshare.com because the disk space on RapidShare.de drives is exhaused, I think it's more related to Google Adsense.
Rapidshare.de allowed users to upload a single file upto 300 MB in size which stayed on their servers for indefinite period and could be downloaded any number of times. Since Rapidshare is not charging non-premium customers, the bandwidth costs were recovered by showing Google Ads on pages that had links to download content off the Rapidshare servers.
However, very recently, Rapidshare.de domain was banned by Adsense since it was primarily used for sharing bulk of illegal content including warez, pirated movies, MP3 songs, etc. The advertising revenue of rapidshare would have take a hit and their next advertising partner, etology.com, may not have been as effective as Google since their ad inventory were just an Adult dating website.
Now that Rapidshare.de has moved to Rapidshare.com, this new domain is already qualified to show Ads by Google. Plus Rapidshare has introduced a new Collector Accounts, like a file manager for all the file that you upload on Rapidshare.com, which could give them tighter control over users and not just the files uploaded on Rapidshare. So if a user is frequently uploading copyright content, they could simply ban the user and all his files are automatically off the rapidshare disks.
The good part of Rapidshare.com Collector account is that you collect premium-points for downloads of your files. These collector points can later be converted into free premium accounts which are otherwise available for 10 Euros per month.