Reactions to buys can be a bad idea because no one expects to have to pause and wait for them so they'll just keep diving into their next buy if they have one. Maybe I spend $10 to pick up Bandit Camp and Wharf, and if I get the Wharf second you'll have to decide on the Bandit Camp without even knowing for sure you'll get a shot at the Wharf, when normally that wouldn't be a concern because you'd pick both up at once. It'd really slow down in-person play. Online keeps things more organized, but it'd also become an issue because okay I buy my Bandit Camp, now the game pauses to give you a chance to reveal a Reaction in your hand, hmm it sounds like you've got a Smuggler's Lair, maybe I shouldn't buy a Wharf right now.
I mean to be fair Fool's Gold has a similar concern but because it can only do one thing rather than different things depending on the card you activate it on, it doesn't seem to cause those kinds of issues. It's a cool concept though.