Anyone's guess. Titanium I don't think was very popular so I can understand that one. Solid white is a puzzler. I guess there is another white, with the pearl white, but don't see how that justifies ditching the solid white. Now there is just one no-charge base color with the black. Less paint options streamlines production, but can't imagine it makes that much difference with the system and automation used.
One can hope they are going to introduce some different/new colors, but don't think anyone knows.