Final answer:
Economics of scale is the factor that did not encourage union mergers. While economics of scale is beneficial, it is not a driving factor behind the decision of unions to merge; other factors like industry decline and jurisdictional disputes play a more direct role.
Step-by-step explanation:
The answer to the question of what did not encourage union mergers is e. Economics of scale. Both economic of scale and avoidance of external controls are concepts that can benefit unions, but they are more related to the operations and effects of unions than to the reasons for their mergers. Factors such as the decline of some U.S. industries, jurisdictional disputes, and changes in the legal environment could encourage unions to merge as they seek to strengthen their bargaining power and adapt to new economic realities. The concern of U.S. laws not being particularly friendly to the formation of unions might also be an external pressure that leads unions to consolidate to present a united front.