several reasons..
1 if they were shipped they go thru a lot to get to you...temp changes jiggling and dropping etc
2 if you had temp issues during incubation
3 humidity issues during hatch
4 it's up to the hen if the egg is fertile...maybe she held out on you.....lol
5 the health of the parent...
you might...sometimes chicks are born with combs and others take a few days to start but in my personal experience it's obvious by 2 weeks or so at the latest