Simply proof that 64-bit wasn't needed by very many people, and even today we're just starting to see mainstream users running into the 3.25 GiB limit.
Remember that I commented on the statement "I can't believe they've gone this long without it" - where "it" is support for large memory systems. I'm pointing out that 64-bit has been there longer on the Windows side than on the Apple side. Those who've "needed it" haven't been going without.