To be fair though, both the Giro and Tour started off in cities, so you had the crowds immediate and lining the route.
The Vuelta began at a racing circuit, so the fans were not so close to the action, because they were in the stands (especially not helped by the wet weather on that day).
Here's a picture from near the end of the lap:
To be fair, some of the shots, especially in the wet, from the far side of the course, show precious few fans, but they filled all the grandstands at the start finish straight.