Because Russia is the sole continuation to USSR according to Alma-Ata Protocol in 1991?

From Just Security and for the subsequent quotes.

“The States of the Commonwealth support Russia’s continuance of the membership of the Union of Soviet Socialist Republics in the United Nations, including permanent membership of the Security Council, and other international organizations.”

And during those days the members didn't want to bring it up because that was the way they wanted it to happened and now suddenly we question their legitimacy because they have turned to be direct threats to us?

The main factor that influenced how the issue was handled in the UN was the basic policy decision of the other P4 (China, France, the United Kingdom and the United States), including first and foremost the US government, which was that it was in everyone’s interest that the USSR be dissolved peacefully and orderly, which could be accomplished if the other republics agreed among themselves on various matters including the former USSR seat and the veto. The republics of the former USSR, including Ukraine, agreed to Russia maintaining the seat of the USSR including in the Security Council. If they agreed, who would object? On what grounds would anyone have objected to Russia continuing the seat of the USSR in 1991? Maybe to get rid of a veto? If so, it was up to a Member to speak up and make the case.

Members were notified that Russia claimed it was not a “successor State” but a “continuing State” with the support of the former republics of the USSR, and there was no opposition

On Christmas Eve 1991 the Soviet Permanent Representative Yuli Vorontsov came to the UN Secretariat with a box in his hand with a new flag of something called the “Russian Federation” and a letter to the Secretary-General signed by Boris Yeltsin, “President Russian Soviet Federative Socialist Republic” (RSFSR). It said “ the membership of the Union of Soviet Socialist Republics in the United Nations, including the Security Council and all other organs and organizations of the United Nations system, is being continued by the Russian Federation (RSFSR), with the support of the countries of the Commonwealth of Independent States, by the Russian Federation.” Note it says “continued” not succession. In the law of succession, he was claiming that parts of the territory of his country had separated, leaving behind the rump which continued the international legal personality of the former larger State, whose name happened to change as well. Same country, just smaller, different borders and a new name and flag. The Russian Federation was the “continuing State” whereas all the bits that spun off were “successor States”—except for, ironically, Ukraine and Belarus which had been deemed as founding members of the Organization in 1945 for reasons not dealt with here. The letter also asked the Secretariat to change the name of the country from “the Union of Soviet Socialist Republics” to “Russian Federation” wherever it appeared.

and...

Members said nothing at the first meetings at which “the change” was reflected.

Although no objections or questions about the claim came in writing, the first time UN bodies met after “the name change” would offer an opportunity to raise the matter in a meeting. Any delegate could raise a point of order from the floor asking “What is that sign ‘Russian Federation’ and who is sitting behind it?” — and thus open up the issue for debate and discussion. The first meetings scheduled after the “change” were not in the General Assembly but rather in the Council. On Dec. 31, the Security Council met for the first time after the “change.” But it was the last day of the month which had heretofore been presided over that month by Ambassador Vorontsov as the USSR representative. On the 31st, however, he presided behind the “Russian Federation” nameplate. The meeting lasted 5 minutes at which a resolution on Western Sahara was adopted unanimously. The President gave a statement at the end thanking the retiring members of the Council. Not one word came from him or any member of the Council about “the change.” The members of the Council who could have mentioned it were Austria, Belgium, China, Cote d’Ivoire, Cuba, Ecuador, France, India, Romania, the United Kingdom, the United States, Yemen, Zaire and Zimbabwe. They all knew and could see there was a change but no words were spoken on the matter. There must have been a clear understanding among members behind the scenes that the Soviet representative would “see out” his presidency for the month as the Russian representative, regardless of “skipping” the alphabetical rotation rule for that one day (S/PV.3025).

Anyway, you can read the whole linked article. It is a good read for those who are interested in geopolitics and the non-bias.