They have a blog explaining this choice here:

https://medium.com/byteball/introducing-a-bridge-between-byteball-and-steem-and-an-airdrop-548d7958b9e9

Here's what I think are good reasons for doing so:

1. Brand Awareness. Think about the state before the airdrop was announced. Barely anyone knew about it. Now people are sick of it. In this respect the airdrop was a wild success. Steem has quite a large amount of users and engagement, and is a logical choice for this purpose.

2. Ease of integration. Via steem connect you saw how seamless it was to attest that you own the corresponding steem account.

3. General Adoption strategy. This is related to (1), but cryptocurrencies need adoption to succeed. They know already that most people will choose to cash out immediately. But we see a clever use of a smart contract that locks your funds for a year which is to say that if byteball becomes successful, it will still have users in a year that may make the choice to stay instead of cash out (seriously you all, back up your wallets redundantly, please).