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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).

Byteball didn't have mining nor an ICO. Byteball team premined the entire supply and decided to airdrop them to Bitcoin holders and also to some other communities.

But they still have about 35% that isn't distributed yet. So, they want to airdrop it in a way that brings value to their coin but also has a fair distribution to ensure decentralization. That's why they decided to airdrop to steem users.

Byteball's team member @punctured recently hinted in a reply to me about that:

All investors in Byteball are aware of the undistributed funds, and that all of it is to be distributed for free. I am sure that when the dust settles, this Steem campaign will have proven to be far more valuable than the initial airdrops to BTC holders. We have already seen impressive amount of content being created and people engaging on Byteball communities. Needless to say, some of the exploits caught the attention of developer communities like thebot.net and Utopian. Having developers become aware of a platform allowing anyone to build on top of it really isnt all that bad.

https://steemit.com/steemit/@punqtured/re-littleboy-re-boladayl-re-littleboy-re-punqtured-re-cryptoctopus-re-punqtured-network-issues-over-the-weekend-and-questions-about-steem-attestations-20180721t201743981z

I believe that byteball is a good cryptocurrency s

ame way steem is also a very good cryptocurrency,i feel they decided to create the special airdrop for steemians because they

respect the steem community,they see a great future in the steem blockchain and they would really love to be involved with the steem

community and also use that as a way of promoting their crypto,well i have not benefited

from the airdrop because i am a new member but i love the fact that many people have benefited from the airdrop,some people even earned as much as 120$ worth of byteballs and that is a good news i must say...

i dont know really but i get messages from different persons that have you collected free 30 dollar coins and i dont know its real or fake.