I obviously have no more of an answer than you do, but are you aware that MtGox is running at a loss? This is probably because they have external private investors who are willing to foot the bill, making short-term losses in exchange for the promise of market supremacy when/if things take off.
The same is probably true of bitcoinica. I don't know who has invested in it, but zhoutong has made references to speaking to investors at various points, so it doesn't seem unlikely that wealthy backers would just decide to cover the cost in exchange for a larger stake in one of the most successful bitcoin ventures yet. The loss may have wiped out bitcoinica's profit, but I wouldn't necessarily assume that just just because they are willing to cover their losses out of pocket that they necessarily made all that money through their own business activities.
Another consideration is that large purchases of bitcoins (such as a wealthy external investor covering bitcoinica's losses) won't necessarily happen on mtgox or anywhere observable to us. I know of several large over-the-counter transactions that occur regularly.