Because it's not just a "Regular part of your rotation"
"Regular part of your rotation" makes it sound like you just toss it out as soon as you hit 3 Greased. Which is a good way to make it a DPS loss.
You need a bunch of requirements met in order to actually gain from using Tornado Kick. Such as having Demolish with enough duration to allow you to get 3 Greased + Twin Snakes before it needs refreshing. Having either Perfect Balance or Riddle of Wind + Snap Punch ready to quickly get back Greased.
Also, ideally, you'd want these things to also happen at the end of Riddle of Fire too. So that you can get RoF's damage bonus onto TK and then once RoF ends you're more freely able to do your noodle damage skills while you build Greased again.
I enjoyed MNK around level 50ish. While in a party.
After level 50, they started adding in all the clunk mechanics and prior to level 50 I did a lot of soloing back when positional bonuses where 40 potency so it felt super awful...
I feel that MNK has such potential to be fun for me, so I put forward my feedback. If SE listen, they listen, if not, then they don't.
However, as things stand, I want to reiterate:
MNK has a unique combo system that doesn't necessarily need to abide by the 123 standard that every other job has. They can be designed to have more reason to mix up their rotations utilizing the fact that their "Stances" open up multiple options for each stage (IMO, NIN has the most dynamic combo purely because while at a Flank Armor Crush will deal more damage than the standard filler Aeolian Edge. While MNK is at the mercy of basically having to flip-flop between Buff and Damage because of such short durations and well... Lack of alternatives)
MNK has their Chakra resource which can be developed on. Heck, if you relate it back to their actual job quests, there should be the potential to go to a 14 point Chakra system, with 7 Light and 7 Dark Chakra to open up and thus allow generation and spending of 2 types of Chakra - Instead of what we have now with the 5 points of Chakra leading to basically just "The Forbidden Chakra" skill.
MNK has stances and Riddle skills to promote being in certain stances. Which, if done well, could actually have stance dancing as an actual part of MNK's rotations (Outside of FoW > RoW > RoW > FoF for Greased generation). Such as if things like Riddle of Earth and Riddle of Wind were DPS CD's like Riddle of Fire that could offset the lack of Fists of Fire.
... Am I missing some skills then?
Since I can quite easily weave all 4 oGCD's into the space of 2 GCD actions without the slowdown. Maybe at high SkS? But even then, given that burst window CD's are 10s it shouldn't be necessary to have slowdown to fit your 4 oGCD's into the time frame even if you only single weave.
Other melee are also encouraged to have high uptime. But don't have annoyingly short durations on their maintenance skills (Also, can quickly get their SkS buff up to make it faster to get the rest of their maintenance skills up. I.e. SAM can get Shifu up as their second skill and NIN can regain Huton via oGCD if for some reason the 70s duration buff ran out...)
Does Tornado Kick reapply all the buffs/debuffs? No.
Meaning that, yes, while Tornado Kick does mimic MCH's Overdrive in terms of frontloading damage before you get into forced downtime, it doesn't alleviate the clunk that is having to re-apply all your maintenance stuff for so many forced downtimes.
Why would it be a bad thing to have some extra complexity into the Monk rotation where you can use an oGCD Form Shift to skip some actions? Especially when Monk's rotations aren't progressively stronger like other jobs (In fact, True Strike is their hardest hitting combo action - Outside of Demolish's full duration that is)
Not to mention that even when things are untargetable, it being oGCD would be smoother as it wouldn't take 2.5 seconds to shift between each stance (Meaning that if you're shifting to get to Coeurl to get an instant Greased stack it takes 5 seconds as you go Nothing > Opo-Opo > Raptor > Coeurl)