This is the biggest thing IMO, liberals would rather attack leftists than try to work with them. All this does, however, is convince leftists that they have nothing to gain from siding with liberals and instead trying to build outside pressure.
Liberals would rather defend genocide than join leftists in trying to force Biden's hand and gain back leftist voters.
I say this as a Leftist likely voting for Biden: Leftists are going third party because Liberals have been blocking all leftward movement, thus alienating leftists from the electoral process. If you want leftist votes, work with leftists, instead of constantly punching left.