If you remember, at the end of season 6 when Riot nerfed the 5-gold champion release rate at level 8, reroll squads continuously emerged and dominated the meta.
The first reason leading to the hegemony of Twitch reroll in season 6.5 is that Ekko was added to the Inventor system.
This is extremely important when Twitch is a 1-gold champion and needs to reroll early, specifically from level 5. Meanwhile, in season 6, you have to be really lucky to own a 3-star Twitch early.
In addition, with the appearance of Tryndamere, you can even switch to building a team of 5 Chemistry Skills – 3 Inventors – 2 Assassins at the end of the match.
Meta also plays a very important role in helping Twitch reroll become the powerful team it is today.
Obviously, such plays will be quite weak in the mid-game (except perhaps for Tech teams), so they become the favorite prey of reroll squads like Twich.
In short, with Ekko’s clan and system changed, the new units supporting him very well and the meta supporting him, the Twitch reroll lineup is becoming extremely dominant in the current meta.
Martin 
Samuel 
Eliana