From distributors in the New York market, there are currently about 450 single malt expressions available. Thus, there were about 2000 more expressions bottled in Scotland last year. If you go to any of the great whisky stores in Scotland, blindfold yourself and pick a bottle off the wall like you're playing pin the tail on the donkey, you have about an 80% chance of plucking something off the wall which is not being imported into the U.S.
The big question: Why is so much good whisky being kept out of the states?
The simple answer: Bottle size and back label requirements.
An enormous amount of Scotch is bottled in 70cl (700ml) bottles. This bottle size is not accepted by the BATF. So unless the distiller, brand owner, or importer/distributor in the U.S. wants to afford the expense of a small and unique bottling run for the U.S. market, the bottles cannot come to the U.S.
For all bottles of wine & spirits for sale in the U.S., there is also the requirement of a back label which, in my humble opinion, requires the unnecessary prose about drinking too much of it minimizing our ability to safely operate a tractor while increasing our chance of getting pregnant. So the same distiller, brand owner, or importer/distributor also faces additional costs for a separate labeling run.
And this is just at the federal level. The importer/distributer must pay to have the front and back label of each bottle approved for each market they want to sell the bottling in. If they want to be in all 50 states, they pay for 51 label approvals - one for each state and one for the feds.
The label situation alone explains why the cost of single cask bottlings are more expensive than general bottlings such as Glenlivet 12, Macallan 12, Glenmorangie 10... Once there labels are approved, they're more or less good forever unless they change their label.
Anyway, any bottling you buy in a 70cl bottle is not something that can come into the U.S.
Edradour, 10 Yr, d'Yquem Sauternes cask finish is an incredibly interesting dram which isn't brought into the U.S. The other thing to look for are bottlings from mothballed and lost distilleries.