Does anyone know why, given the demand, the Shigaraki teapots are so seldom produced and more expensive? Is the clay rarer or harder to find etc.? Just from within the forum I notice a semi-continual interest in the pots.
"given the demand" perhaps here in with lies the key to your question.
Hojo is arguably a marketing genius. At the very least he is an expert on supply and demand as well creating very enticing product write ups. I kind of doubt the clay is that rare or hard to find, it is however Hojo's personal selection that only he commissions for use in conjunction with Masaki-San. Hojo also has a very clear sense of "taste" if you follow his products and tea selection you will see it emerge. His tastes have fallen in favor with the general teachat crowd and show a valid influence, though not to say their are those who do not find his tastes preferable amongst this forum.
Now combine that with the fact that many Teachaters hype these pots. I myself am one. I own two of these pots and would only consider another one if the clay came out in a new shape such as a hohin, or perhaps a yuzamashi.... Yes, I would defiantly get a yuzamashi. I feel a lot of his claims take license of poetic language, but these pots seem to be rather substantiated in their claim to fame. I have tried a selection of nearly every type of his clays, and for me this is the hands down winner when it comes to sencha. Masaki-San is also an excellent craftsman and his work is supremely functional.
while I believe no simple answer to your question is adequate, these pots have become a sort of "holly grail" and will likely only increase in perceived value and desirability.