Why did SE decide to make the three new classes not require base classes?

[Sylve]

I guess I just love the lore, and would hate to see that stripped from the game.

Its not really stripping anything.

The Class to Job progression in Lore is the Adventurer rediscovering lost Magic/Skills.

Going forward, our Jobs are new to the world of Eorzea, so having a Job Crystal that whispers secrets of old techniques makes absolutely no sense, lore wise.
These aren't Jobs that existed in the distance past, So having no base class makes a lot of sense.
One could argue that the Classes are whats left of the Jobs as the more advanced techniques were lost to time.