Because it still contains some blood? Not exactly, I shall say rather because it contains a protein called myoglobin. This protein, located in

muscles bring it oxygen. It has the same role as the haemoglobin of red cells. It is an intermediate transport between blood and muscular grain. Oxygen fixed on haemoglobin breaks loose to go on the myoglobine which distributes it to muscular cells

Everybody knows it : blood is red and this is owed to the haemoglobin contained in red cells and which fixed oxygen. The myoglobine has a structure quasi identical to haemoglobin and she also gives a red colour when it fixes oxygen.

In conclusion, the red colour of the meat is owed to the presence of myoglobine having fixed oxygen.