There were some assumptions why it is so, shortly written most popular of them are:
-They are a new team, not established in the fans minds.
-They haven't a popular driver like Lewis, Fernando or Robert
-They are branded based on a drink
Maybe there is something in this third point? I mean, car producers, had/have a pretty big fan base, because their cars have some fans. Privaters team, gain easily fans, because their are usually the under-dogs, and it's pretty admirable, that some private guy want's to race in the pinacle of motorsports and make a challenge to the big car companies.
And there are many people who would love to drive a Ferrari or McLaren once in a live, there are many people who are fans of Williams or Sauber because that are personalities.
And now look at a drink company? Can you be a fan of a drink company? Can your dream be, to own in the future a big can of this drink? That doesn't make sense.
So IMHO it's not a matter of time, rather a matter of the brand itself, altough I can be all wrong here.
And naturally I didn't take into account the second point, I mean, if Robert would drive for RBR, IMHO I and really much much people would became RBR fans from the scratch
