I don't think there's just one single reason why the hashrate doesn't drop on a price dip. These are some of the reasons that might contribute to the phenomenon:
- a miner running an asic farm can't switch pow algo's, so he doesn't have a lot of choices to start mining on a different chain. As long as he produces more btc than his hw cost, he"s probably better off if he keeps mining... Even if he can't roi.
- some amateurs don't care about profit, they just mine to keep the diff high
- ignorance
- people that keep mining because they believe the price will rise again, but don't realise they're probably better off if they'd stop their mining and buy btc directly instead of paying more for their power than they generate in btc