Final answer:
The major reason for the decline in the viewing audiences of the three major networks is the growth of cable television.
Step-by-step explanation:
The major reason for the decline in the viewing audiences of the three major networks is the growth of cable television. Cable television provides viewers with more options and specialized channels that cater to specific interests. Additionally, the expansion of the internet and the availability of online streaming platforms have further contributed to the decline in traditional network viewership.