Here's why:
* "The Winged Dragon of Ra" has a special summoning condition: It can only be Special Summoned by tributing 3 monsters. It cannot be Special Summoned by any other method.
* "The Winged Dragon of Ra" restricts other summons: It has the following effect: "This card cannot be Special Summoned except by tributing 3 monsters. When this card is Special Summoned, no other monsters can be Special Summoned for the rest of this turn."
This means that once you Special Summon "The Winged Dragon of Ra", no other monsters can be Special Summoned for the rest of the turn, regardless of their summoning conditions.