Your toe nails are placed under a lot more pressure then you finger nails. so they need to be more robust to handle the presssure.
to discourage people from biting their toenails !
To be firm footed on the ground the toes need some strong grip. Hence their structure is such. Moreover the entire weight of the body is always rests on our foot. so along with the progressing years they become stronger and stronger up to some age.