Because Ethereum blockchain is ideal blockchain for mass adoption and easy to deal with. Even Microsoft has started building their Azure platform on Ethereum blockchain and IBM is working on it as well. Ethereum blockchain hold way too much potential to turn around things in way better and faster.
It's all about complexity and Simplexity. In fact, coinbase already considered to add ERC20 assets after the coinbase audit has given a result about ERC20 assets is stable assets.
You can see that on the latest news of coinbase to open the possibility to add ERC20 assets to the GDAX platform before it will be listed on the coinbase platform.
And in addition, there is a lot of demand for ethereum assets rather than another asset.