Why is Premade so hard? What the hell am I doing wrong?

I used to be a high-plat to low-diamond player, but ever since DQ was introduced I've played almost exclusively 2-4man premade queues and since then have dropped from Plat 2 to Gold 4 in less than 150 games. Every once in a while I play a streak of solo queue games just so I can climb back to Plat, but immediately after I go back to playing premade and my MMR tanks to ~Gold V even though my rank doesn't show it.

I can consistently carry my solo queue games, but I cannot win premade games for the life of me. Every premade game just feels like a super uphill battle where everybody, myself included, just feeds relentlessly to silvers and golds. Of the handful of games we do win, most of them are long 40+ minute games where the other team just throws at some point.

Why does this happen? What is the secret???