Check out this site. It will most likely answer your question. http://coinresource.com/guide/photograde/pg_$1Ike.htm
The reason is that they didn't want the damn thing to cost too much. Strangely, the first mint set with an Ike in it was 1973, and none were made for circulation.
That kind of defeats the purpose of the mint set, doesn't it? Was there an actual source to get uncirculated copper-nickel clad Ikes straight from the mint in 1971 and 1972?
I presume there's a story about why the 1999 proof sets didn't have a SBA dollar. Does anyone know about that? I know that the US Mint made a number of SBA dollars for collectors that year. Leaving them out of the proof sets seems kind of short-sighted. :kewl:
