Any product decision in Live Audio will be made for the following reasons (not necessarily in order of importance): size and weight, cost, reliability, performance.
Why aren't 21" drivers used more often for subwoofers?
1) size and weight - 21" loudspeakers are HEAVY and need a bigger, stronger box - this impacts load crew and truck pack. The one you show from P-Audio weighs in at 68 lbs just for the driver - about double the weight of most 18" speakers
2) they cost significantly more than the 'industry standard' 18" drivers or boxes.
3) reliability? couldn't tell you as I've never owned one - maybe someone else has some facts and figures or a personal experience. I would have to say though that they have yet to prove themselves in the market.
4) performance - I've heard several people argue that 21" subs are not the nicest sounding boxes - they lack clarity or punch. Sure, they go low, but they don't give the sound that most live audio engineers are looking for.
There ya go . . .