I have to wonder if what changed was FX technology and budget.

It probably wasn't that hard for them to add the shield shimmer effect that would indicate "a shield is here" or "the shield is under stress because something like a phaser is hitting it" to various scenes in post-production in TNG, which meant that when you said "there's a force field" the viewers at home could see the force field, have a sense of its presence and effect.

That sort of stuff was a hell of a lot harder in TOS, when saying "there's a force field" would often mean either "take our word for it, it's there" or additional and potentially tricky work in post.

-Merc
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