There's also the matter of visibility. Characters in animation need to be easily distinguishable on sight. Putting them in spacesuits would've made it harder to tell who was who in a long shot. The force field belts let viewers see the characters just as clearly as they could normally.
I'm not sure that is entirely true. Hanna-Barbera's Sealab: 2020
(1972) regularly featured characters in the oft-used "duty" uniforms, but had the smae characters in diving suits where the face was covered save for the mask, whether it was a close-up or wide, and this period of H-B animation was as cost-cutting (already produced out of America) as other U.S. animation houses. That said, I don't think TAS risked breaking the budget or confusing viewers with TOS-styled spacesuits.
Certainly, the force field version took less labor to execute, but this was a series where a host of new aliens, TOS characters, etc., with elaborate costume designs were the rule, not the exception.