User blog comment:Justin Holland/The Flash Season 4 Theories/@comment-4503935-20170918015709/@comment-24152851-20170918015934

In-universe, Barry probably hadn't gained Thinker's attention at that point, or possibly Thinker didn't even have his Thinking Cap yet so didn't have his villain resources.

Out-universe, because the writers hadn't thought of him before season 3.