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It is already being used as a payment system. It is the adoption that you are trying to hope at.
Questioning about why other Bitcoiners to sell is that it is their choice to do so.
In the end, they are still using bitcoin maybe not as what you think it is being used by other's envisioning.
Investors will always think of profits as their topmost priority. Whether they want to sell or not, it's always their own choice when they can make good profits. Bitcoin has already achieved its goal to be used as a currency in some part of the universe and for others while they are waiting for the adoption of bitcoin in their countries, they can make use of bitcoin to gain the best profits possible as long as bitcoin continues to be the best asset or investment.