How do promotionals work?

Anyone else who has gone 7/10 in promotionals or better getting stuck in brozne 1. If so post here i want to know why I can duo with my friend all 10 matches win 7 of them and i get stuck in bronze 1 and they get put in silver 2. This system is so unfair. Its like 10x harder getting out of bronze 1 to silver 5 then getting any other rank thanks to the horrible quality of players in that tier. So yeah fuck you riot this shit is depressing.