That's just a question of how many kind people there are who compile, pack, test (!) and submit extensions. Doing that can be a lot of work, especially for younger penguins like me.
The fact that the 64-bit repo has less extensions just shows, I guess, that there are probably more people interested in / using the 32-bit variant of Core. One reason could be that, while Core Linux is not designed to be used on old hardware, It's still a good choice for resurrecting those old boxes.
I did submit every extension I made when I thought it's usable enough for others. Just started playing with Core64 today (just to see what happens) but will probably not switch to CorePure64 in the near future, since I don't really see the benefit for my case ( < 4GB RAM and not too much storage for 'redundant' installations on my x86_64 Desktop - my Netbook is x86 only).
The 64-bit era has arrived but it hasn't redeemed the 32-bit era yet.