Question about Kennedy Half Silver Basic Set

Can anyone explain to me why the 1976 Silver Kennedy is not part of the Kennedy Half Dollars Basic Silver Set, Proof (1964, 1992-Present)? Is it simply because of the composition not being 90% silver?
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Comments
Kennedy Proof Set
We'll use our hands and hearts and if we must we'll use our heads.
I would have to guess that it's because it's not 90% silver like you speculated.
We'll use our hands and hearts and if we must we'll use our heads.