That was explained by the PM Nikolai Denkov

Bulgaria has been forced to remove the tax imposed on the transit of Russian gas to Hungary and Serbia. That happened after the Hungarian Prime Minister Viktor Orbán threatened to veto the country’s entry into Schengen, Bulgaria's PM Nikolai Denkov said before the meeting of the European leaders in Brussels.

"Hungary has sent a message that if the fee is not removed they will veto our Schengen membership. We decided that there is no need to risk at this time with important, but still not decisive steps in terms of Schengen," Denkov underlined.