The other day Donald said that in general, on-gain is more fun than on-buy, so he generally only chose on-buy when there was a good reason (Noble Brigand being on-gain would have caused things like Jester to have to be played in slow-motion, for example). If hoard were on-gain, it would be slightly more powerful, but there's only a handful of combos where it would make any difference. (Black Market, Horn of Plenty, and Farmland, Border Village... Any others?) of, and it would be less powerful with Trader and Posession around. I doubt that version of Hoard would be overpowered... So why not on-gain?