I just read this article, in which former director Kraft is described as deriding NASA's current plan to use a single heavy-lift vehicle (Space Launch System, or SLS) as a primary launch platform. Why is NASA doing this?
A single failure destroys an enormous amount of work. Distributing the across rockets increases the possibly of a single failure, but a single failure results in less loss. Furthermore, the smaller rockets exist today, so it doesn't require a huge new new development effort. Since they exist today, a lot of the design flaws have already been discovered and corrected.