I can imagine a variety of reasons (ok maybe just a few) that 24mg is the highest offered.
24mg may be the highest that Don is willing to go due to quality of flavor...some juices seem to taste better with more nicotine, but some certainly seem to suffer. If you cannot tell, Don prides himself on his juices and I can imagine he doesn't want to release something that isn't to a certain standard (or have polar opposite reviews due to nicotine content).
I only use 18mg, and have not ordered anything in 24mg so I can't give MY personal experience with 30+mg nic or anything close, but it may be that Don feels anything higher than 24 would be a liability issue. Although it would be a pain to have to add more nicotine to every order, for those looking for higher nicotine content it is a viable option to purchase liquid nicotine of the desired strength and DIY (IF YOU KNOW WHAT YOU ARE DOING).
Don charges the same amount for a bottle of 0mg as he does a bottle of 24mg, while other companies may charge slightly more for high nic liquids. Sure, he could put a dropdown menu on the website if cost was truly the issue and charge a buck or two extra but that would involve even more customization and special orders for an already busy man.
All of the above may be false, but I was just providing a little speculation =D