I found this story on sportscollectorsdaily.com a while back. It talks about the 1970 Kellogg's set and how it didn't include Hank Aaron and Carl Yastrzemski. It doesn't get into details as to why these two are not included. If you know, please pass along that information. Did Topps have an exclusive deal, did the players not agree to a contract with Kellogg's, did Kellogg's not want them?
For Aaron, he appeared on Topps cards through 1976 but didn't make it into any Kellogg's set until long after his career - 1991. It would have been nice to get a 1973 and 1974 Kellogg's card of Aaron when he was chasing the home run record.
Yaz didn't appear until 1975, so his first Kellogg's card is (a) more expensive and (b) usually cracked and curled. Lots of these 1975 don't display well.