Except WordPress is almost completely founded on its plugins, so that's a non-trivial consideration. Specifically, if WordPress cannot provide proper abstractions, sandboxing, and protocols for plugins to be secure by default, the issue could be greatly reduced. As-is, its model both encourages such flaws to be included and provides its non-technical users with no viable way to identify which are likely to be vulnerable plugins.

Sandboxing would be nice, but I don't think it's practical. Do you know of any secure plugin systems that can replace WordPress?