Because lets say you decide "hmm, I'm going to parry this projectile" and then do so. When the next rock comes, you will likely also parry it. Now given all 6 rocks have different speeds after leaving his hand, how will you know when you use your parry? Answer would simply be to see where the rock is and time your parry to it, or just block.
Now you have the issue that not every rock will be the same rock or the same timing, and there is a second option in his ground shake that prevents blocking or parrying from being an absolute answer. To avoid this ground shake, you'd have to jump, but what happens when you can't jump because of rocks on the screen? How will you know when to jump, or if to jump, when you're also devoting your mental time to figuring out when to parry or if you should be blocking that first rock.
To quote the old Bane days "Knowing what you need to do, and actually being able to do it, are two different things." Its easy to say "its obvious when Bane will do delayed armor/DP/Command Grab, just punish it", but that line of logic doesn't function outside of a vacuum, at least not consistently. If you play a good Tremor, one that actually knows how to zone decently well, you will find yourself having to guess. Its something that no amount of MU knowledge can prepare you for. I'm not saying one side or the other wins, however to assume that you don't have to guess against a character who can delay or cancel 3 different projectiles which have different speeds and trajectories, and then has another 3 projectiles all of which have different timings and properties, and each of those changes with each variation... Just doesn't make logical sense to me.
The problem is making meter to do 4.31% as chip (or like 15% on hit) isn't fast or efficent enough when Tremor is slapping your block for 2.54% with each rock, slapping your shins for 8% every unblockable, and so on.
That being said, as I've said before I am not a Kotal player (not much of a tremor for that matter). I play Ferra/Torr and Shinnok. I contributed what few cents I had, but I'm far from what could be considered a reliable source of high-level opinion.