Why do deep-sea divers use a special mixture of gases and not just air to breathe?

Well, for starters, air is 79% Nitrogen/inert gas which will cause the decompression sickness if breathed at pressure over time as nitrogen levels build up in your body, then expand as you come up. This is known as the bends - following the Recreational Dive Planner tables, you can avoid staying down too long on normal air, but the tables for gas mixtures with higher oxygen levels give more time. However, oxygen becomes toxic at 1.4-1.8 bar/atm(atmospheres of pressure) which is about 15 feet down on pure oxygen. This can cause convulsions, which are bad on land but really bad underwater. So, finding a balance and knowing your depth and time limits is critically important. Adding other gases like Helium can extend both depth and time, but get very expensive very fast and require years of training.