They are slow and it seems to me also very susceptible to adverse conditions. So if the potting mix dries out, or they get too hot, or they get too cold, or they damp themselves out before the seedling gets above the soil, or they get eaten by tiny birds before you ever see them, or a house
mouse grazes them flat, or or or or...
After years of failure I have finally managed to successfully start pepper seeds a couple of years running. I do it the Joseph Stalin way: I overrun the enemy with an endless wave of cheap troops. "Quantity has a quality all its own." Instead of planting one or two seeds in every cell, I plant eight or ten. Instead of just planting the three varieties I might want in a given year, I plant
three similar types for each variety, for a total of nine sets. And so forth.
Basically I still get massive failures, whole sets of starts that never do anything or where just one seedling germinates. But somehow this way I still end up with half a dozen pepper plants in my garden.