From the fact that Princess Luna spoke Early Modern Ponish (thee-and-thou Shakespeare Ponish), not Old Ponish (Beowulf Ponish—see pic), we know that Old Ponish had already ceased to be the vernacular 1000 years ago. That's why the Pillars didn't speak it.
Why didn't Nightmare Moon say thee and thou? Because the evil purple goo that had possessed her was modern
Why didn't the Pillars say "She hath" and "Thou art"?
Because...because...ah, the hell with it. I dunno.