Most frozen veggies are frozen right away, so they are pretty damn close to fresh in terms of nutrients. Some of the "fresh" ones have been sitting there so long not preserved anyways, and are downright rancid.
Frozen is usually better than fresh. No sitting around in crates and being trucked all over the state to your local grocery store to sit on a shelf for a couple days before you buy it and bring it home and wait a couple days to eat it.
Today's modern refrigeration enables people to consume veggies that have been picked at the peak of freshness, and then preserving that freshness indefinitly until the veggie is used.