@BudXs, every sorts of reason really. As I barely had the time to germinate them, they stood there without any care appart from minimum watering.
So there was pythium, fungus gnats, sun burns, a couple died from heat and drought...
When I could plant them outside, even with the fence, some cat or bird unearthed some of them, foraging for worms (lots in my compost), some plants didn't recover.
Overall, the early stretch was manageable in itself, it was just an indicator that they were not at an ideal location, but I couldn't do anything about that.