I'm not sure, but I suspect it's because it has the most liquidity (trading pairs, trading volume) after bitcoin, and much lower transaction fees.
Another factor could be that lot of people already have Ethereum accounts, and ETH, given it is the most widely used cryptocurrency by daily transaction count. Many people have an Ethereum wallet just to be able to buy and hold ERC20 tokens.
It's not just Ethereum. I have seen this scam aimed at DGB and MAID folks as well, and those are just two that I pay attention to, so there are doubtless many more.
Another factor could be that lot of people already have Ethereum accounts, and ETH, given it is the most widely used cryptocurrency by daily transaction count. Many people have an Ethereum wallet just to be able to buy and hold ERC20 tokens.