Hi cryptkeeper, first off, I hope you becone a regular commenter since this is the type of active engagement I hope to receive :)

Now the response:
You actually raise similar points as I did when I had a talk with MEDIA Protocols team yesterday. I can see why they won't use SBD; there's some speculation that SBD is backed by $1 fiat of STEEM so it could eventually be pegged at that price. Even if the chance is low, a 3rd party project won't take that risk. I actually asked them why don't they use STEEM blockchain.

I learned that the main reason is that any URL (and later IPFS address or other distributed protocol for sharing content based on blockchain technology) can be registered against an Ethereum account and form part of a transaction between content publishers, promoters, distributors and consumers.

And if the team feels they eventually need more scalability, they stated, "We have the option of migrating the token to a different blockchain, which could be our own, or another third-party alternative to Ethereum."

It's good that they have considered all possibilities and are prepared to migrate to alternate blockchains (or maybe even build their own) if the need arises.