I think this is nore an issue of "reasoning" rather than "reason." The CD format holds about 74 minutes of music. An LP holds about 45 minutes.
Clearly, you are correct. Vanguard could have loaded the individual CDs with considerably more music. The reason why they didn't might be one (or both) of the following:
1. It's less profitable to do so, and/or
2. Many people think of a disc as a program. 74 minutes is a long program... not an ideal program length for some people.
All the best, Dan