The demise of key changes
When looking at every Billboard Hot 100 number one hit between 1958 and 1990, we see that the key of G major was a very popular key. This was because the key of G major is easy to work with on the guitar and piano, the two most popular compositional instruments during these years. In fact, across the decades, we see that keys that are convenient to use on these instruments (i.e. C major, G major, D major) are more popular than others that are less convenient to use, like B major and Gb major... More here.
Cannot beat a good key change and it's a shame there are fewer examples today.