It was likely a strategic decision that was guided by the fear of civil war, domestic terrorism, foreign terrorism ( Russia), and specific threats to some of their families. Keep in mind that trump, some of his cabinet, and congress people are being blackmailed and threatened by Russia
Biden and his intelligence agencies had to have been preparing to for what they knew was coming once they decided not to challenge the election results of which they may not have had enough evidence to prosecute in the court of law and public opinion in the short time before the inauguration
All of this is to say that they’ve been waiting for Trump to go before announcing this all. Once the cult leader is dead the cult and Putin lose power over the country. A Russian asset stealing an American election is the biggest bombshell in American history. Can’t hide it for much longer.