Well, I'll make a completely uneducated guess here...
Amazon (owner of mobipocket) saw the stunt Google and Sony did with their massive ePub library and wanted to do something similar so they went to PG and asked them to provide their library as .mobi files.
This step makes sense in more than one way. On the one hand they have a large library of well-formatted books (not as many as google, but probably less errors) and in addition (an this is where the conspiracy theories come in) they start profiling .mobi as their DRM free PD format (which you are welcome to use on the Kindle) and start widening the gap between .mobi and .azw (or whatever the Kindle file format is) that is their commercial format, available on Amazon only and useable only on the Kindle.
Something to think about perhaps?