When "small screen" access was restricted to a tiny, technical, minority, it simply wasn't sound to invest any time, energy or resource on building and maintaining sophisticated mobile friendly websites.
Of course, in recent times, this has all changed and the traffic served to mobile devices is often exceeding that served to desktop machines.
It's become important to have a website that conforms and adapts to the vast range of platforms and screen resolutions available today; so much so that Google now looks at and evaluates the responsiveness of websites and gives added weight to those that are "responsive".