The reception of a Ukraine or a Georgia into NATO would obligate the United States to go to war on behalf of a country that is of no meaningful strategic significance to us and that can offer no meaningful assistance to us should we be attacked. If Ukraine had been a member of NATO in 2014, the United States would have been obliged to go to war with Russia over a territory that Ukraine made absolutely no effort to defend (in fact, over 9,000 of their soldiers and officers and over 7,000 of their civilian administrators defected to Russia). What benefit could possibly accrue to America from such an arrangement?
To be clear, I don't mean to take the side of the government of Russia over that of the government of Ukraine. I'm simply arguing 1) that the relationship between these two states is far more complex and intertwined than most Americans imagine and 2) that we stand only to harm ourselves by entangling ourselves in their affairs.