Pre-fork ETH owners didn't necessarily have to pick one over the other. Miners had to allocate their resources between the two chains (but could split their bets, and some did so). Pre-fork ETH owners ended up with both chains in a kind of 2-for-1 special.

Every market transaction since then has been a referendum on which model the market prefers. Ultimately, the market price dictates a LOT of down-stream behaviors, e.g. some miners will switch back and forth between ETH and ETC (and other alt-coins) based on hourly yield.