If you have multiple 12v panels in parallel, they are actually 18-19 volts or so. Your non MPPT controller sets it at about 14 v to the batt at however many amps from panel size.
Shade just one panel and your voltage can drop to ~10 volts and you get almost no amps from the charger.
Now wire the panels in series and have ~60 volts or whatever your MTTP charge controller can handle......one panel is shaded and you end up with maybe 30 volts, but you are still over 14v which means you still get amps from a MPPT, in a nut shell.