Because Object is a special type of Variant, so requires a different handling from the other variants types. Also all the other variants (except from null, see the list here) have constructors to create a new variant using (), like Vector3() (see the Vector3 docs). So basically, any variant that inherits from the Object class needs the .new(), the others have their constructors to be created with the ().
Just wanted to add that neither Object nor Vector3 are Variants. Variant is a container class that can hold Objects and Vector3s and any other Godot type. Check the documentation for info about what Variant is.