I dont know why this happent,but probably something wrong in the incubating progress killed them.
Was this your first incubation? i incubated once, 3 eggs, 1 lived and the 2 others hatched but they were both crippled and died 1 and 3 days later in my case my humidity was low in the first days, and the thing under the shell got dry and thicker and that was my issue.
The other chicken lived. I enjoyed its life so much, it died few hours ago!! It just got sick and died in 2 days
RIP 14 February - 31 August 2011
I wish you good luck with your next hatch, dont worry you will get over it, dont stop trying and get new eggs in the incubator after making sure the temperature and humidity controls are correct. I use 3 different temperature and humidity counters.