Biden's son had just died in the spring of 2015, and he was very distraught over it - wears rosary beads ((not a microphone lol)) on his wrist to this day in remembrance of him. i think this is why he didn't put in a bid then - he has run several times in the past, and a VP is a natural candidate after any 2-term president.
Clinton has aged and given the way she was demonized by the R gang beginning in 2015 and still to this day, i'd say her chances weren't great of winning - so why spend the tens of millions of dollars it takes to become any kind of serious candidate when all she would be doing is essentially siphoning support away from another member of her D gang.
Gore didn't run again even tho he had actually won the 2000 election