When Googling this question, I typically get that this was due to a new social class of teenagers. However, with more restrictive laws against child labor and better standards for education, teenagers were around long before the 1950s. So my question is:
As to why a specific thing suddenly becomes dominant, well, who knows? Changes in fashion so often hinge on a particular individual in a particular period of time. A style that might have been around for years with only a minority of people interested can suddenly become wildly popular for no clear reason.
But there were a few things in previous years that, if not specifically to Rock 'n Roll, were leading to changes in the way music was happening. First you have technological changes. Advances with electronic amplifiers mean that you don't need a full orchestra to deliver enough sound for a dance hall - in the 20s, to get his trademark soft vocal sound in large halls Rudy Vallee used an acoustic megaphone, with amplifiers by the 30s crooners like Bing Crosby just needed a mic. And this was applied to instruments to, especially the guitar - fairly quiet in comparison to other common band usuals, but with electric amplification by pioneers like Les Paul it becomes transformed. Secondly there were problems with the musicians unions in the 40s - the strike from 1942-44 over royalty payments, and again in 1948, in the long term led to the demise of the 'big band' orchestras. Vocalists and small (non union) bands saw their style start to grow in popularity as they were basically all that the record companies could release at times. Lastly you have the post war world (and now we're starting to get into psychology more than history) and the everything is assumed to be changing in the new, technological, everything-is-changing brave new world.
So, at this point in the late 40s early 50s, you've got both the recording industry and listeners that are pivoting to smaller bands, a demand for something new, and at the same time the post-war boom means it's also much more affordable for small groups to enter the business with only a few instruments and a lot of new ideas, and still fill the room with sound. As so the rock 'n roll style won - but really it might just have easily have been a slightly different style of music, if the fashions of the day had led that way instead.