Because this is an offshoot of something else we’ve been building using the Simplicity scripting language that officially launched last summer. Liquid already has all of the features we need in terms of speed and reliability, and doesn’t have the liquidity and liveness constraints of Lightning. It's intended to be a seamless UX for users with Boltz swaps built-in, so deposits from Lightning and on-chain are easy.
https://blog.blockstream.com/simplicity-launches-on-liquid-mainnet/