The dispersion of launch costs has shrunk over the years. A new paper noted that three effects could be causing this phenomenon.

Historically it has been documented how there is a high degree of trial and error in the early application of technologies, leading to a wide dispersion in productivity outcomes.
Space exploration began in a Cold War era and was dictated by the objective of showcasing scientific advancements. As a consequence, launch costs were a secondary concern, especially with respect to reliability. Spacefaring nations would want to launch using their home-based rocket technology irrespective of the cost.