Originally, they were the first ones to come up with complex smart contract abilities. Bitcoin now has them, but they are still much easier to develop on Eth. There are like 100 other altcoins that support contracts too. But Bitcoin is really catching up in this way and there's no reason to think long term that it doesn't surpass Eth's abilities (because it can just absorb features from eth). At that point, the question smart contract developers will face is, why use Eth when Bitcoin has a larger user base, larger liquidity, and higher decentralization/security? There'd be no reason to pick eth. Eth is becoming increasingly centralized, I would stay away if I were you. Like every coin except Bitcoin, they traded decentralization for speed.