In general, the water-soluble vitamins are the most susceptible to damage by boiling. Vitamins A and D will survive heat. Most of the B group leech into the cooking water. Vitamin C is very prone to being useless after boiling.
To answer your question, it is because vitamins are destroyed plus also the fact that they leech into the cooking water. You cannot recover Vitamin C from cooking water but you may be able to recover some of the others.
Try microwaving vegetables with a couple of tablespoons of water in a heat resistant covered dish. This remains the best method of retaining vitamin content other than by eating the vegetables raw. Steaming is also a good alternative.