I don't think it's anything to do with supply. Supply in general is massively overstated as an effect on coin prices. The only thing that matters is what's on the actual exchanges.

I'd say the main reason is that ETH has evolved into not much more than an ICO generator and ICOs are about to get a walloping from multiple angles. If it isn't regulators going after them, it's the people who poured millions into shite that will deliver nothing.