Because all of the money and users that you hope use your killer mainstream app have to come from somewhere - a brand new blockchain makes your app that much ha

Because all of the money and users that you hope use your killer mainstream app have to come from somewhere - a brand new blockchain makes your app that much harder to launch. In addition, your killer mainstream app may only be useful inside of an existing ecosystem. Ex: Uniswap is pretty useless if you don't have any assets that people want to trade.