(follow up question to scimonster's question)
Shushan Purim was enacted because Jews in walled cities stopped fighting on the 15th and thus that's when they celebrated, opposed to the rest of Klal Yisrael who celebrated on the 14th (see Beis Yosef, Orach Chaim 688:3).
My question is: why don't we follow rov (majority)?
I understand that people in walled cities were still fighting on the 14th, but since the majority of Jews WEREN'T and were instead celebrating, shouldn't that majority be enough to kovea(establish) that Purim be on the 14th for everyone?