Genuine question: what's the difference between yogurt armor and Founder's Primes?

With the hubbub around FOMO I've seen a lot of people cry foul about the FRV, and a few use DE and Warframe as examples of what to do right.

FRV aside, some are also bringing in the pre-order armors as examples of "bad" FOMO, since they can't be gotten anymore. Warframe has a very good comparison point in the Prime sets that were sold once and never again, but why is that given a pass while the blue armor must be released again? Could Arrowhead just release a purple set with the same passive, like the Excalibur Umbra?