The reason is you're not meant to come out ahead after promoting a post on here. The true value of your post is what its organically deemed to be worth by users of this platform.

This place isn't worth a thing if people are paid to advertise on it and sellers get higher returns than curation to support them.

For more information, read my main comment here https://steemit.com/bidbots/@trafalgar/pxcvtq