You do realize the Left have refused to admit that they lost and Trump won for the past 4 years.
It may be helpful if you tell us who you mean by "the Left." Who exactly are refusing to admit they lost the election to Trump in 2016?
When Trump was first elected, a good number of people took to the streets to protest. Some of those wore shirts and carried signs saying "Not my president." Also visible were the "never-Trumpers" on the Republican side. Both these groups were fiercely critical of everything Trump did or, in their eyes, failed to do. Then there were the House and Senate Democrats who voted to impeach Trump (the first time). They too were fiercely critical of Trump; so much so the took extraordinary action to impeach.
But in none of these cases did they challenge the election itself. They noted - correctly - that Hillary Clinton won the popular vote but acknowledged Trump won the Electoral College vote. They did not say the election was stolen from them. They did not say the election results were a lie. You may be able to find a few exceptions to this but those would be rare. There was no organized stop the steal movement.
Yes, Hillary said "illegitimate" in the article you posted above. But nothing came of it. Her supporters did not continually take to the streets to protest the legitimacy of the election. They took to the streets to protest Trump. That same article shows that. Most of what Hillary was quoted as saying was not about the election, it was about shortcomings she said Trump demonstrated in office after he was elected.
And about questioning the legitimacy of a properly elected president, what about the continual barrage of tweets and groundless accusations by Trump about Obama's citizenship? Trump was playing the delegitimize-the-president game long before he became a candidate himself. Now he is lying about the legitimacy of Biden's election results.
Out of respect for the tradition, former First Lady Hillary attended Trump's inauguration, thereby acknowledging the legitimacy of his election. Off that stage, she offered several reasons, including voter suppression, for her defeat. A far greater number of people on "the Left" offered that Hillary lost because she was a poor candidate who made several strategic blunders.
By attending the Trump inauguration, Hillary supported the peaceful transfer of power from one administration to the next. Trump is not doing that. He is not going to attend the Biden inauguration. He tried to thwart the transfer of power by unleashing a mob on Congress as they were counting the Electoral College votes. And he continues his attack by encouraging those who are willing to use violence to achieve their ends. Washington DC is now fortified against such people; people Trump refuses to call off, and people he continues to encourage by promoting his big lie about a stolen election.
Trump is being impeached for that. He may be convicted in the Senate for it. He will likely be criminally charged and prosecuted for it after his presidency ends on Jan. 20.