I think it would be really hard to identify exactly why this happened, but I am highly doubtful fear about the medium term possibility of Arena showing up could be the reason. Magic players just don’t think that long term about things, especially limited players. They draft, sell their cards, and then draft again.

I can’t really say what the reason is exactly. It’s possible that AKH block draft was more skill intensive than KLD block draft, and this resulted in better players winning more prizes. If more prizes are concentrated in the hands of the best players, they might be the ones selling more boosters into the secondary market, which results in the price dropping lower. That is just a guess though.