Mainly it was because there did not appear to be any precedent of vendor-specific multipart types.
That's technically true but irrelevant. THere are no special restrictions on who can register multipart types. There is, however, a requirement (RFC 4288 section 4.2.6) that any type registered under mulitpart MUST conform to the multipart syntax described in RFC 2046. If your type uses, say, a zip container or something similar, it cannot be registered under multipart.
These composite assets are intended to represent a single logical asset comprised of multiple assets. An example of such an asset would be the Photoshop Elements file format "PSE", which is a single file that contains multiple assets much like a ZIP file that in turn is registered under application/zip rather than multipart/zip.
The real question with container types is whether or not they meet the requirements of RFC 4288 section 4.1. Most do but occasionally one comes along that does not, and when they don't no registration of any sort of is possible. Ned