Why climate alarmists are so fond of using the late 1970s as a baseline

From this article:

[The 1970s] was an unusually cold decade, especially the later years, across the Northern Hemisphere. In the USA, the winters of 1977-79 were three of the 11 coldest since the recording of temperatures began in the 1890s, according to climate center data. The winter of 1978-79 remains the coldest on record in the USA.