Why should diclofenac sodium not be given to people who have high blood pressure?

Moderate
NSAIDs (applies to diclofenac) hypertension
Moderate Potential Hazard, Moderate plausibility.

NSAIDs including topicals can lead to the onset of new hypertension or worsening of preexisting hypertension, either of which can contribute to the increased incidence of cardiovascular events. Blood pressure should be monitored closely during NSAID therapy and throughout the course of therapy.

Source: https://www.drugs.com/disease-interactions/diclofenac.html#hypertension