They weren't produced for very long, and there may also be a combination of cultural attitudes (old toys may get thrown out instead of sold at thrift stores and gararage/boot sales) and the fact that people who have them don't realize they are valuable collectors' items to the rest of the world.
Exactly this! And from friends/family here, I was told for cultural reasons equine themed toys weren't very popular in Japan in general. It's too bad, I think they're adorable, and I'd love a Takara to go with a Takara outfit I scored way back.
*edit* darn spellcheck turned way into wary, lol