Pashinyan answered why he chose the EU mission instead of the Russian CSTO.

During the government hour in the Armenian parliament, Prime Minister Nikol Pashinyan was asked why the country chose the EU mission rather than the CSTO to monitor and resolve the situation on the border with Azerbaijan.
Pashinyan said that Armenia asked the CSTO clarify its area of responsibility, but received no response. Representatives of the European Union clearly outlined the territory of Armenia, and came to monitor it. 
Earlier, Russian Foreign Minister Sergey Lavrov said that the plan for the deployment of peacekeepers had been proposed to Yerevan, but the Armenian side wanted to categorically condemn Baku’s actions. This would prevent the CSTO from resolving the situation.
According to Pashinyan, Yerevan is ready to open railway and automobile communication with Azerbaijan as soon as possible. This will significantly increase Armenia’s trade with the EAEU. He noted that the country is interested in unblocking regional communications.