Thought Ruler is a great synchro, but at the time this list was made, it simply didn't quite make the cut.
Ruler's second ability (which can negate a spell/trap that targets a psychic monster) isn't as useful as it may seem. It can't negate popular cards that don't target (Raigeki, Mirror Force, etc.), requires the target to be psychic, and you must spend 1000 life points to use the trait. Definitely still one of my favorite synchros, though.