There many reasons why a caterpillar might not survive to adulthood. It can be very difficult to determine exactly what happened to a particular larva, but these are some of the reasons one may die.
There are many diseases and parasites that affect monarchs. Infection can be transmitted between adults and/or larvae, especially if the monarchs are in close proximity. For more information on the risks of rearing monarchs in close proximity to one another, see our handout: Rearing Monarchs: Why or Why Not?
The genetics of each monarch can also determine the outcome of each monarch, some may have a genetic reason they did not survive.
The contamination of milkweed can also reduce the odds of monarch larvae successfully making it to and survive adulthood. If the milkweed the larvae ingest carries pesticides, bacteria or other harmful organisms, this may also be a reason monarchs might not survive.
Monarchs also have many predators that are part of their natural ecosystem.