For several reasons I'm guessing. Notice how f0xy chooses not to end the majority of his combos into hard KD? Since you can't tech roll in MK11, the value of hard knockdowns is now less than what it was in previous NRS titles. In certain situations, depending on who you talk to, it makes more sense to either restand or end in regular KD because of the numerous options players have on wake up that make it risky to continue to pressure outside of the corner. Because of the distance f12 creates on hit, and the length of recovery on hard KDs, when you use it anywhere outside of the corner it gives your opponent enough time to recognize your approach and wake up accordingly.Is anyone else triggered by Foxy's open refusal to utilize f1? I can't knock a guy who's thus far the only Lao doing anything with the char at a tournament level (3rd place is hella impressive), but Jesus. I'm actually blown away he does as well as he does considering. No, f12 is not confirmable but it's a decent stagger, safe on block, and you can condition people to respect the followup and hold the f1 way more than f4, which in offline play you have no legitimate claim to stealing your turn back after anyway.
Another is because it's not special cancelable. It's -7 and you effectively give up your turn against any player who know's the frame data and how to take their turn. It doesn't create space either so it also leaves you susceptible to throws which puts you in a guessing game the moment it's blocked. F4 is -3 and hit confirmable/special cancelable which gives you some breathing room for mind games and better pressure for oki with z hat when you use it as an ender. You can cancel into orbiting hat (or spin if you're a maniac) & condition your opponent to hesitate when taking their turn, or you can flawless block reversal OS the low poke that usually comes after.
Lastly, it might be because f1 isn't really a true stagger. It's -5, and if I stagger into another f1, that creates a 15 frame window for my opponent to do something. If I stagger into d3, that's still a 12f window. Now against players with weak mids/bad connections/poor reactions/lack of frame knowledge or all of the above, that's an abusable gimmick. But against players like Dragon, NinjaKilla, etc. in an offline setting? Not so much. I've come across good players who like to press buttons after f1 because it's full combo interruptible with not as much risk as you'd think. If I f12 they risk 6% into a hard kd and the wakeup/oki guessing game begins. If I f13 and they block the f1 but get hit by the 3, I won't be able hit confirm it into anything unless I commit to the spin which is risky. This is mostly the case for Hat Tricks, but in Lotus Fist you can cancel into hat and get the confirm off the 3 reliably at the cost of 2 bars & a 2f link.
That said, I agree that f0xy should've used anything other than just f4/throw as a follow up to the z hat frame trap. By limiting himself to only those options, his opponents only had 2 things they had to look out for and you could see it wasn't enough to throw off either Dragon or Sonic. I realize that setup is the closest thing to a 50/50 we have, but if you're able to consistently react & guess right on the grab then f4 becomes less effective when your opponents start relying on techs instead of neutral ducking.