The biggest factor is almost certainly laws in many countries including the US which shield anyone and everyone involved in the making and distribution of a vaccine for COVID (or other pandemics) from any liability whatsoever for any and all consequences of the vaccine. Even if, to take it to the extreme, it kills people and they knew it would kill people and still sold it, they still can't be found liable in any way.

Other comments are correctly pointing out some corollaries of this, such as trials proceeding very quickly and trial phases being run almost in parallel; but the root cause all of this can happen is the legal immunity for the consequences.