Another posted indicate code issues, but I think you're right this is at least a big reason. GPG is GPL licensed, but GPGME is LGPL licensed. A library version of GPG would have to be GPL licensed as well, which would require any programs linking to it to also be GPL.

With that, for GPGME to be LPGL there can't be an actual coupling of code between the two or else GPGME would have to be GPL licensed itself. (Though you could argue the distinction here a bit murky)