My guess would be because the 1973 Ikes were not minted for circulation, and were only available to collectors in the mint sets that year (incidentally, the first year they were available as such). As many of the mint sets have since been broken up, it's not impossible--though not common--to find 1973 Ikes in circulation...I've pulled 1973 P, D, and S (clad proof) all from circulation within the last year or so.
S holes are probably not included in the Whitman folder because, again, S minted Ikes were not made for circulation. They only made the silver uncirculated and proof coins and the clad proofs. If it makes you feel any better, they also left out holes for the Type 1 Bicentennials from both mints. Space was probably also a concern since they crammed the Ikes and
SBA dollars into the same album and there wouldn't have been enough space to include anything more than they did.
I looked at sample pages for the album you mentioned and, interestingly enough, they DO include holes for the 1981
SBA dollars, which were also only available in the mint sets. They include S mint holes because in 1979 and 80, San Francisco did mint them for circulation, with 1979-S being more common than 1980-S.
If you're interested in putting together a set of
Ike dollars, I would recommend Dansco album #7176. This is what I have my Ikes in. It also includes holes for the uncirculated S minted coins in the years that they made them. And the Type 1 Bicentennials too. It's a bit more money than the Whitman folder, but so very worth it!