I can think of 2 reasons that a tank wouldn't work as well. First, the shape of the hatching container matters. In a tank, the unhatched eggs would just settle to the bottom (in the corner away from the bubbler), and they wouldn't get the aggitation they need to hatch. In an inverted bottle, there is nowhere for them to settle, they keep getting stirred by the bubbles. Second, BBS have the most nutritional value during the first day or two right after they hatch. So while culture 1 is at that "perfect to feed" stage, the culture 2 is in the "working on hatching" stage. Two days later, culture 1 is "too old to feed", but now culture 2 is "perfect to feed" and it's time to get culture 3 started...
In this context, the word "culture" is kind of a misnomer. The BBS aren't growing, they just hatch, swim around for a few days, and then die.