For all we know, forcefields are less prone to catastrophic failure than mechanical suits. It's inconceivable that a forcefield would get punctured, for example. Just install a failure-proof power pack there; Starfleet must have plenty of those in stock, or else its starships would go kaboom at random when antimatter containment hiccuped...
Well, they don't trust their force fields so much that they don't put doors on their shuttle bays. As for catastrophic failure: Unless you run into the Squire of Gothos or some such, a space suit can't just be turned "off". Further evidence that they aren't such a swell idea is that both before and after TAS they used suits.
Some ideas from TAS caught on (holodecks, unfortunately) some didn't.
Didn't Space Academy use some kind of life support belt?
will actually re-animate TAS in 3D or live action or whatever. It would be interesting to see if it could be re-edited to eliminate the belts.
Obviously Once Upon a Planet is way out of it's league on this one. Maybe we SHOULD match up Beyond the Farthest Star and One of Our Planets is Missing.