If one considers all the factors, whether in a sealed or vented sub, a pair of 12's will have about the same output and extension as a single 18. It's easier to put the 12's into an elegant cabinet and it's cost effective as well.

Axiom could easily have an 18 - but when building drivers, building 2000 twelves at a time is more cost effective than building 1000 twelves and 500 eighteens.