It may have something to do with intellectual property rights. After the revolution Belaieff publishing house left Russia and went to Germany. I read somewhere that the Soviet government applied their export limitations also to musical editions. So maybe all the scores have not been allowed to be published in the west and only the old Russian editions were available, now probably out of print. Just a guess of course...
Scriabin died in 1915. Unless he wrote actual real life Opp Posts anything he did could have been taken by Belaieff with them.