Many of the newer commercial varieties are developed with an emphasis on tenderness and short DTM. I would guess that it is the variety grown. The older storage versions like Mammoth Red Rock are somewhat tough but do do have a much stronger flavor than modern hybrids.

It's likely not a frost issue because that usually brings out more sugars, not less.
I think it's a combination of variety, soil conditions and possibly because it's been sitting in cold storage for awhile.

If you buy it at the farmers market again, ask the vendor when it was harvested.

You might have just had the bad luck of a couple sucky heads of cabbage too. I've had it happen with produce where suddenly for a couple purchases it wasn't good as expected- regardless of where it was bought from. Then just as inexplicably, it was good again.

The dense heads are the sweet ones. I pick mine by how heavy they feel relative to size. A really dense head is usually good. The light ones aren't worth buying. I'm pretty sure cool growing conditions lead to the denser, sweeter cabbage.