I've used Magic Launcher a little bit, but when I did, I noticed it was way more daunting to a new user than it needed to be. MultiMC is great for a new user, because it's fairly simple to use, less stuff to mess up, especially when you have someone telling you exactly what to do. You don't need to tell them how to download and create a new Minecraft instance, or how to put it in a new folder and select that jar, or what an "External mod" is versus a "mod."
"Make a new instance. Click on Instance. Click on Edit Mods. Put these mods in 'JAR Mods', these in 'core mods', these in 'mods folder.'"
Much less for someone who isn't familiar with the inner workings actually doing that means.
Don't get me wrong, Magic Launcher works just fine, but I'd much rather tell someone how to set up a MultiMC instance than that. Or heaven forbid tell them how to do it manually.