I think it's because of how this game jam was graded and scored. Gameplay is only one criteria in rating the game. Yes, the gameplay in this game was quite fun (gameplay was your highest rated criteria, after all!) but the judges rate on more than just gameplay. Things like how you implement special object and theme matter too. For example, some games may not have had as fun gameplay as yours, but if they implemented the theme and special object better they would get a higher score.
Your rating on a game jam doesn't always reflect how fun or enjoyable a game is; all it means is how closely the game aligns with the judging rubric. Don't worry about a low rating—people have clearly stated they enjoyed your game.
In the end, you made something fun to play, and I think that's what's most important!