Because, well an example:
"Download my amazing app: It's Steem without the annoying promoted posts!"
There is no direct incentive for UI developers to make their app worse by including the promoted content. Work on this issue and you might have something.
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If an app excludes promoted posts it makes it better -> less paid spam.
WRT: changing the steem homepage to promoted posts.
Yes, it would make promoted posts more attractive, but it doesn't make steemit more attractive to users. If a new user coming first sees promoted posts and then has to click to get trending or their own feed then that's a turn off.