Because when you touched it, you heard a tone

The Awl speculates and reminiscences about the touchtone telephone. 

When Your Fingers Did The Walking:

On this day fifty years ago the people from Bell Telephone (another one that I guess you had to be there for) brought out the touchtone model, which was another thing we used to have before you’d swipe your germ-encrusted fingers on a glass screen to make contact with someone else who you are probably only texting with because who talks to each other anymore? Anyway, you could press buttons to dial a call! It was AMAZING.