The vast majority of keg beer from most breweries comes in 11 or 22 gallon kegs. Casks for trad beer have never caught on in those sizes so are always, with few exceptions, based on the traditional measures of 9, 18, 36 and even 54 for some breweries such as Joseph Holt. The relatively smaller sizes are better for selling the contents of individual containers in as short a time as possible.
I've never seen a Sam Smiths 9 gallon cask and I doubt the cooperage would bother making or repairing them so if you don't sell enough cask OBB to justify 18s then it doesn't go in.