If cooking doesn't kill all the nutrients in tomatoes, why are none in sauce labels, even the nice ones made direct by an organic farm?
I would swear that there was already an answer about this on SA, but I can't find it.
Pressure canning destroys a lot of the vitamin C value of tomatoes:

The heating process during canning destroys from one-third to one-half of vitamins A and C, thiamin, and riboflavin. Once canned, additional losses of these sensitive vitamins are from 5 to 20 percent each year.

(there are other papers with more scientific information, but they're all behind paywalls)
This means that the finished jarred tomatoes in a year-old jar might have only 30% of the original vitamin C content -- and that will vary according to a lot of circumstances that are hard to predict.  So, unless you test ascorbic acid content of the finished product over a lot of batches, you really don't know how much C you have, on average.
While the US FDA does want you to list C content on the label, you are unlikely to be punished for under-reporting the amount of C ... whereas overreporting is a lot more likely to get you an unwelcome phone call.  So many canners simply leave it out.
For that matter, many commercial canneries report zero vitamin C as well.  I have several cans of Muir Glen tomatoes in my pantry, and none of them report C content.