Hi,
Based on my extensive experience in killing plants, I think you have a bacterial blight. Perhaps if you cut through a stem, you might see damaged tissue. These tend to be soil or seed based and usually need high humidity to get going. Maybe try cleaning the seed in a dilute (very) chlorine bleach solution or with 3 percent hydrogen peroxide for 10 or 15 minutes and then rinsing in RO or distilled water. I don't guarantee results with this, but I do it with pepper seeds with no problem. Also if you weren't already doing it, sterilize the media. Tomatoes and peppers are kind of sickly. Tomatillos on the other hand tend to outgrow their diseases.
Good luck
