Q: Why doesn't BB design more subs like the VS21 hybrid box? For example, having a "VS18"?

It comes down to a balance of practicalities. The VS21 is bigger than a direct radiator 21" would be. Any 18" cabinet with a hybrid horn would be bigger than its direct-radiating counterpart. The larger mass, and therefore greater inertia, of a 21" cone introduces limitations on the acceleration of that cone. The result is experienced as a lack of "impact" or "punch". Other descriptions are that 21" woofers are "slow " or "muddy " compared to 18" and, to a larger extent, 15" cones. The hybrid horn produces higher sensitivity in the upper end of the subwoofer operating range. That higher sensitivity correlates with a reduced demand for excursion in that range. With a reduced demand for excursion comes a reduced demand for acceleration, thus overcoming the generally perceived limitation of the larger 21" cones being "slow ". While some heavier, large voice-coil 18" cones suffer from this limitation, it is to a lesser extent, and most 18" cones can be accelerated fast enough to not need this secondary advantage.

The net takeaway is that building such a cabinet for an 18" woofer doesn't yield sufficient improvement to warrant the added size, weight, complexity and cost of its construction. People would rather have an 18" loaded cabinet that is smaller given that the performance of an 18" woofer in a direct-radiator type construction is not otherwise compromised. We therefore ask ourselves, "Would a single 18" cabinet that's significantly larger and more expensive than the SSP118 but not as loud as the VS21 tend to serve anyone better than either the SSP118 or the VS21?"