For me, it was the fact that all ten people had the same champ that made it boring. I'll give two examples:
Game 1: All Blitzcrank. Pulls and knockups and silences all around. Same build on each player. Very little fun and drawn out. (Loss)
Game 2: All Lucian. No pulls, minor escapes, no real initiation. variety in build. Was fun having sound effects that made it sound like I was in a Star Trek shootout. Still, drawn out and unentertaining. (Win)
In both games, we ran into the same problem. Nobody could initiate well. With blitz, it was just getting picked off one by one. With Lucian, it was just spamming abilities and AAs.
In both, it just felt like such a grind to take objectives. Getting one objective didn't give any real advantage because the attacks and tactics were the exact same throughout the entire match. You knew what was coming and you knew how to either avoid it or counter it. So did they, and it led to a boring, drawn out match.