Why not the Top 50? Or the Top 75?
Top 40 makes no sense. So you are in a band that records a catchy song and it makes it to number 41 in the Billboard Hot 100. Not a shabby achievement. Yet it never really was a hit because Casey Kasem never introduced it on the radio. Not even a sappy long distance dedication from a knocked up waitress named Gladys Knoblock in Joplin, Missouri to some truck driver hurtling down I-10 someplace in Tucson, Arizona desperately trying to get the heck away from Gladys. (Song was "Don't Give Up On Us Baby" by David Soul.)
Why you might not even have bothered recording the song at all!
The Top 40 is just so...arbitrary. A kick in the stomach for recording artists that fell just short of it.
I need to know...why the Top 40?
Because that's how many songs were in a radio stations playlist. 40 new songs from the top of the charts. That way as soon as a song dropped out of the top 40, a newer song that was maybe number 48 the week before now entered the Top 40 and would get airplay. 100 new songs are too many to play at any given time what with re-currents (last years songs) and oldies to play too. If a radio station played the top 100, you would never hear your favorite top 40 song (the ones that grab the most listeners) more than once a day.