The reason is they hot fill their tanks and why for god's sake do they not have nitrox?
I am not sure of other ways of mixing nitrox but getting food to the island is tough enough. Getting enough O2 tanks there would be much more difficult. Also, with normal dive profiles being go to 40m and look down into the blue, there is a fair chance of a lot of people exceeding MODs and if I were the operator, I'm not sure I would be keen for divers to use it there either.
There is no excuse for underfills though.