Mainly just the size.Cerwin Vega might have limited availability on this, but I have the SPCL182 Stroker 18" dual 2 ohm in the 2017 catalog. That's a $1599 sub, plus shipping.
A beefy 15" is a lot easier to find. What do you like about an 18" subwoofer that a 15" can't offer, other than the bigger diameter?