My answer-
Straight up: It’s versatility sucks. It’s only good for large scale destruction. Granted, Eren in titan form is unable to do anything but hold on.

[insert gif of attack titan being pushed by colossal titan] (couldn’t find one lol)

Against smaller units, it’s cumbersome and slow, making it hard to land a blow as people with ODM gear move much faster.
It’s body is also too large for someone to support for an extended period of time, and every time it uses its steam, it loses muscle mass, which in the end, is what got Bertolt killed.

Not to mention its arms are freakishly skinny. The colossal titan is gigantic and destructive, but it’s a one-trick-pony, so to speak.

Keep it classy,