The nature of the charts could have been influenced by a producer. There must have been situations in a which a producer asked for something "jazzy, but not too wild or out there," and the arranger provided standard dance band fare.
By contrast, the 20th Century Fox album I noted above is clearly designed as an easy listening album, but the charts are pretty interesting. You can pay attention and not be bored, or have it on in the background and not be distracted.