I would assume that they simply didn't manufacture enough of those single sided sets to meet the demand. I also seem to recall some issues with Season 10 having some bad discs, which may be why that season is extra hard to find. If this truly does get a Blu-ray release, my wallet hopes that...
I bought the original double-sided discs, I bought the reissued single-sided discs (expect Seasons 10 & 11, which I STILL haven't found!) and I'll be ALL IN for the Blu-rays too!