Why does the PlayStation 2 only have two controller ports?
To keep the cost of the system down. Sony used to sell each PS2 at a loss (so did Nintendo and Microsoft), so they had to cut corners to save as much money as they could. These days, they could make it with 4 controller ports and still make a profit, but it would be a waste of money to go back and re-design a system over 10 years old.