I guess that's because people haven't got accustomed to her moves yet.
As I mentioned in one of the posts a few weeks ago, her 3PP/3P2K isn't that threatening.
People are not going to throw out 3PK in neutral unless he wants to do something different. Besides, 3PK only causes a knockback with damage no more than 40 and is unsafe on block so I don't even consider it.
3PP causes a knockdown with a small amount of damage, too. If one ducks to try to block lows, he gets free damage from a low throw punishment and only loses a minor amount of damage if hit by 3PP when ducking. Same can be said about retaliating immediately with i11 mids if a character has it. So on paper the relative safe option is to just block her 3P2K.
I think you're really underestimating how good Mai's 214P and variants actually is. It is a i12 frame tracking mid punch that is a knockdown up close and a lift stun for +32/+23 a bit away, as well as a legitimate launcher on counter hit (6PKK or 3PK juggles) at the same range. The knockdown up close will give you an OTG you cannot gain from other fast mids in the game (or at all from a single hit fast mid). Which is the ability to completely take away the WUK from the opponent, you can force them to tech up because of buffering in microdash 1K. No other character has this from an i11 or i12 mid off a single hit (correct me if I'm wrong).
I've been abusing the crap of 6PK knockdowns as Ayane for a long time, I'm abusing the crap out of Mai's 214P knockdowns the same way. You get to keep up the momentum and pressure. You don't lose either at all.
Mai's 3P2K is also very good as a range tool (keepaway, low poke, space control from 3P). You have to be wary of 3P, 3PP, and 3P2K against Mai at range because her 3P is a legitimate footsie move. Has more range than her 8P (1.56 vs 1.55cm makes a difference IMO, I can feel it as I play) and both moves are great options with good follow ups.
All of 3P's follow ups will knockdown in critical stun, neutral hit and counter hit, except for 3P2K, which will only do so on counter hit and critical stun.