It takes a year to hatch because by the time you had eggs, the growing season for mulberry leaves has ended... next spring, when the weather warms up, they will hatch.
The silkworms are very much in tune with the seasons.. they won't hatch unless they 'know' that there will be ample food to eat.
Same with the animal kingdom.. they don't breed in general when there is a shortage of food, or when they can't afford to be gravid given their own individual circumstance.
I was once a suggie breeder. You had to give them lots of protein in order for them to breed, in order for their offsprings to survive, basically, they breed when time is good.