Popularity, likely one distributor has control over the exporting them from China, and in the past they had low mintages.
Curious as to why you've decided to invest in bullion. There can be a lot of reasons, good, bad and in between. BTW, I'm a coin collector, but don't consider it an investment. It's a hobby, and with luck, I won't lose too much. Cal
I to have swung in to this direction of bullion investing coming over from the currency side, though I would not consider myself a coin collector rather a coin Investor along with rounds, bars and such have been focusing on gold as of late but still trying to increase my Silver supplies.