I think that the "cool factor" for many classic Spidey villains is insufficient to get them into these movies.
So far as we've seen and heard, the Spider Man movies are hero flicks, but they focus on telling the story of a young man struggling with life as a hero. Action is secondary to the story, thankfully. A villain that does not make that story more interesting or complicated really need not apply.
Rhino, Scorpion, Electro, Vulture, Shocker and Sandman are, for the most part, simple fighting villains. They make for great action scenes, but nothing about them screams "I make the world more interesting for Peter Parker." You could write a story around them, but they have few plot hooks in and of themselves.
The Lizard, Kraven the Hunter, the Black Cat, Mysterio, Green Goblin II/Hobgoblin, and/or a well set-up and constructed Venom all have greater psychological possibilities as story elements.