Helmut Marko told to 422race.com that they choose Buemi over Alguersuari because the Swiss driver was better fit for the simulator. Interestingly a Spanish newspaper quoted on the 9th of January RBR Tech Chief Adrian Newey telling that RBR offered Alguersuari the possibility for the same spot but he turned it down!
I have looked around and the indications were that the story was incorrect. I contacted also the team and I got the following reply : "Reports in a Spanish newspaper suggesting Red Bull Racing chose its third driver after Alguersuari turned down an offer from the team are incorrect. No one from Red Bull Racing has spoken with this paper and the quotes attributed to Adrian Newey are completely inaccurate and false."