Because the Democrats spend more time fighting leftists, then they do fighting the Republicans. Check out the recent DSA elections: Democrats spent a bunch of time and energy fighting against them.
Additionally, the Democrats fight tooth and nail to keep third parties off of the ballots. They also spend a bunch of effort saying that things like "not bombing kids" or "medicare for all" are unreasonable, or unrealistic, to keep people from thinking that 3rd parties are viable.
