Final answer:
The decline in the volume of spam email since 2014 can largely be attributed to the enhancement of spam filters and heightened cyber security awareness among users.
Step-by-step explanation:
One reason why the total volume of spam email, as a percentage of all email traffic, has declined since 2014 is due to the improvement and implementation of more advanced spam filters and email security practices. These technological advancements have become more sophisticated in detecting and blocking unwanted emails, thus reducing the amount of spam that reaches a user's inbox. Additionally, there has been a growing awareness of cyber security threats among internet users, leading to more cautious online behaviors and a demand for greater email protection, which has helped to decrease the prevalence of spam.