SpaceX is the only medium-heavy lift provider in the West with free space on their manifest right now. Sure they're constantly launching Starlink, but their entire offering is that if someone else needs to launch a payload, they'll just repurpose a launch that would otherwise be carrying Starlink (since external launches come with a profit, while Starlink launches are at internal cost).
Vulcan's capacity for a year or so is already booked, and Ariane 6 is also fully booked for several years out. Others in the class are approaching first flight, but still lacking enough information to book a launch on.
Ariane 6 is booked for years, SpaceX use most of their launches to put Starlink satellites in orbit. They have a lot of capacity to sell more launches, actually.
Vulcan's capacity for a year or so is already booked, and Ariane 6 is also fully booked for several years out. Others in the class are approaching first flight, but still lacking enough information to book a launch on.