My guess...there's not much reason to trade BitUSD if it is intended to be stable. Also at this stage it seems there is not enough growth in new demand for holders of BitUSD to see much growth in supply, which would need to be facilitated by opening of long/short trades. This suggests the incentives are not high enough to attract longs given the still experimental nature of BitAssets, and that its all new and early days.
I think if there were a market for lending and borrowing of existing BitUSD, allowing interest to be paid from borrowers to lenders, this would give greater ability for trading and opening up of new supply. The reason is that those with a positive view on BTSX and want leverage, who currently can only short BitUSD, must compete for the very limited number of longs willing to open at the feed price. And longs do not have enough incentive yet to do so, and thus limit expansion of the supply. With a lending/shorting market, those lending will find a return in the lending market, and rates can also be locked in, which keeps them simple and transparent. Those shorting will find liquidity there at the interest cost, so both parties are now economically incentivised to open up new positions in the long/short market at the feed price. I suspect the interest rate would be more attractive at that point, possibly 20% pa +.