Answer to Question 1
Because they occur deeper in Saturn's colder atmosphere, below a layer of methane haze.
Answer to Question 2
Images from spacecraft reveal that Io has over 100 volcanic vents on its surface. The active volcanoes throw sulfur-rich gas and ash high above the surface. That ash falls back to bury the surface at a rate of a few millimeters a year. This explains why you see no impact craters on Iothey are covered up as fast as they form.