If you look at:
UK instrumental number ones : https://en.wikipedia.org/wiki/List_of_instrumental_number_ones_on_the_UK_Singles_Chart
US instrumental number ones : https://en.wikipedia.org/wiki/List_of_Billboard_Hot_100_chart_achievements_and_milestones#Instrumental_number-ones
You can see that in the 50s to the 70s, there were quite a number of instrumental number one singles in the UK and US charts. Since then there have been very few. What could be the reasons for this?