The "promotional" rate fails attracting new users. And even if it succeeded, people who just buy some HBD and stake it in Savings would hardly become users, they would just take profit once in a while.
I wrote down reasons for lowering the APR; and the post has an interesting discussion with 100+ comments. I don't want to brag, but I believe it is worth reading (and so is the discussion, obviously), as the interest rate should be one of the keen topics for the Hiveans ;)