The process of getting airplay for songs is the most basic form of sales in the music business, and it's done on several levels. It doesn't just happen. The fact that a song is on a national chart is an indication that the process is happening, and it's having success. That success will be used to convince other stations or outlets to add the song.
The people involved in this process are record promotion people. They are the warriors for music. They work for record labels or artists, depending on who owns the song. They call and visit radio stations, tastemakers in the media, playlist curators at Apple, Spotify, and Pandora, online radio operators, program directors and producers at Sirius and national radio syndicators, and anyone who can influence airplay.
National charts can be programming tools, just as they can be marketing tools. The fact that a song is in a national chart means it gets airplay in the national chart shows like American Top 40 with Ryan Seacrest. So even though a local station has not added a song to its local rotation, it will get a spin on the nationally syndicated show. By the same token, it might be getting airplay on national online services and satellite radio. The artist might go on national talk shows like Fallon or Ellen to perform the song, and get fans to request the song at local radio. Local requests can get attention. Social media is used to drive attention and requests for a song.
Music promotion people are the direct link between the artist and airplay. They can deliver the artist to the station for interviews or promotional concerts. At the core of the process is the single, the song chosen by the label to be the focus track. The airplay chart follows the song as it gets added by more stations around the country, and as those stations increase airplay from light to heavy. Depending on the format, that can mean anywhere from 10 spins to 100 spins a week. That kind of attention builds familiarity in new music. It's not usually instant, and it's all voluntary. There will be some stations that will never play a song, even if the artist is a major star, and the song is in heavy rotation and in the Top 5. That's just how things go. Some people will never add a certain song, just as some people will never vote for a certain candidate no matter how popular that candidate is. He might even win the election, and those people will never support those candidates. That's what makes the world go round.
There are lots of factors in a decision, Recently there was a popular song in the national top 10 that had a lot of sexual innuendo and negative language aimed at a former lover. For some people, it was funny. For others, it was perceived as nasty, bitter, and even abusive. For that reason, the song was controversial, got a lot of negative research, and never made it to #1, even though the artist was very popular.
In the end, it's a process, it's a decision, and there are lots of tools used to influence that decision. Everyone has an opinion on what is great music. The point of programming is to play what the majority of people want to hear. Over the long haul, each of those decisions will determine how popular the station is. Airplay of one song won't make a big difference, but the more bad decisions a station makes, especially in a competitive situation, the more likely it will be that the station will lose listeners.