Question : why don't some 32 bit programs work with 64 bit os?

I know some don't work, and I am not looking for a way to get them to work, I just want to know why this happens from a purely academic standpoint.

Actually a few friends were asking me this the other day and I could not come up with an answer.

Answer : why don't some 32 bit programs work with 64 bit os?

Here is a restatement of what has already been said. It is describing XP 64, but is still related.

    http://www.microsoft.com/windowsxp/using/64bit/russel_exploringx64.mspx#EYE

 (under "Key Differences" section)