It doesn't make sense to me that you can't raise someone from the dead after they died from venom, when you can if they died from getting their head crushed by a dragon's foot.
Like most fantasy, DnD health seems to be based on the idea of Élan vital, with Constitution reflecting how much 'life potential' one has. Certain things, poisons and others, lower that amount. If your amount equals zero, then you have no life in you. When a dragon smushes your head, your life potential remains the same. Think of it as a clay jug of water siting in a bowl, with the water representing the life potential, the jug the body. Smash the jug, the water is still there. The jug can be fixed or even replaced. However, remove all the water,and it is now just a jug. You can patch that jug all you like, but it no longer contains water, the body no longer contains life. That isn't how life works in the real world, where life is the workings of an nigh infinitely complex machine, but it is basically how we think of it instinctively, and it certainly simplifies things like healing.