I don't know why other than that being an Inc-front end change and not a backend thing. Keeping lists requires centralization, and how would the consensus form? I think that Steemit and @ned needs to step up and rename the index.html to trending and rename promoted to index, then it will remove all excuse for using bidbots for promotion and those that run them will face the music.
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Accounts that have large or many delegations can be suspect as a first filter to look into. Then, transactions can be looked at to see if they receive transfers often and upvote same people who send the transfers. This would identify a vast majority of bots. Checking for refunds/returns is also a cross validation. All it takes is some activity to catch a bidbot account.
Changing the default front page is something that should have been done a year ago... :/