Biden and Sanders are getting up there in years. 2020 could be their last chance.
On the other hand, Kamala Harris, Spartacus, and others, are still young in presidential years. They may well sit out 2020, expecting Trump to get reelected, and wait for an open election in 2024 with no incumbent.
“Biden and Sanders are getting up there in years. 2020 could be their last chance.”