It is a mystery to me too. Whatever the reason, I suspect that it is not concern for humanity.
I had the 1st Pfizer shot in Jan. I skipped to 2nd shot due to a bad reaction. So I got the J&J shot in June. Before the J&J I watched ‘quack’ videos on the vaccine-induced Deep vein thrombosis (DVT) and the headaches which signal its onset. I never used to get headaches but started to a few days after the J&J. Now, thanks to the ‘quacks’ I take aspirin every 4 hours, drink lots of water and take 3+ mile walks whenever possible. I suspect I will have to continue this routine for possibly a year or whenever they start calling for a J&J booster.