The question isn't why the fund stopped buying.  The fund buys and sells bitcoins in direct response to people buying and selling shares of the fund.

The relevant question, which Silbert probably has insight into, is is why people aren't buying into the fund as vigorously these days.  Perhaps they have exhausted their list of people likely to buy, and they have scaled back their cold call operation.  Perhaps the months of price stability has made it a hard sell.  I'm guessing that once we get back over $1000, fund sales will pick up again.