This is because the ModelMesher is Mojang's code and must be called at a very specific point during startup, whereas the ModelLoader is Forge supplied so that you (as a modder) can't fuck up.
Coming from someone who updated all of his projects quite recently, I actually had a lot of trouble trying to update. I can understand why they are confused. I'd offer to help, but my "patchwork" is shoddy on that bit. I often just update ItemModelMesher on my old mods to match the current syntax, and just use ModelLoader on new projects.
ItemModelMesher works, but it doesn't work well. ModelLoader is a lot more stable.