I think it makes perfect sense, for the same reason that Buttigieg is the second choice for a lot of Warren supporters even though they support vastly different policy prescriptions. Maybe I'll do a longer explanation later, but the simple answer is that they are cut from that same Aaron Sorkin-esque Left-Liberal post-ideological cloth, in which politics isn't about Left or Right, and it isn't about principles or values or vision. It's about data and statistics, and the idea that if I simply sit you down and explain the data comprehensively and eloquently enough, you will be unable to bridge any exceptions to the fact that my policy prescription represents the one true, rational, objective, correct answer. It's why they all fetishize "bipartisanship" so much. Politics seen through this lens isn't a pitched battle of fundamentally incompatible ideologies or divergent ethical frameworks, but aesthetic choices which are different only on the superficie, and which rest atop a positivist, empirical, data-driven Base.
Politics, then, isn't a battle of ideology or vision, but one of expertise: who has the best data, and who is the most qualified (and therefore most likely) to evaluate and apply that data "correctly" into pragmatic policy prescriptions. The policy prescriptions, then, between Kamala or Klobuchar, or Buttigieg, or even Warren, become superficial and aesthetic choices. The only question worth asking becomes "who is the most credible?" And the answer to that comes from rattling off line items on a CV: is it Warren, the Law Professor from Harvard, or Buttigieg, who was educated at Harvard and Oxford, speaks 7 languages, and put his data-driven analysis into practice as a mayor who "transformed" (snerk) a languishing Midwestern town, or is it Kamala who made rational, data-driven decisions as Attorney General of California/San Francisco to produce positive economic growth and improved social outcomes.
Framed in this way, it's no surprise that a) Trump, a doddering, populist, deeply ideological buffoon, and a supremely unqualified, data-illiterate (and possibly functionally literal-illiterate) twit to boot represents an existential threat to these people; he's anathema to everything they hold dear, and b) that the universal cry of lament from the Hillary-stans was that she was "the most qualified candidate in electoral history," and, that the fact that she failed to win election has nothing to do with her policy prescriptions or her rhetorical style, and everything to do with, alternatively: stupid, racist Midwestern and Appalachian voters who "don't know what's good for them," "Bernie-or-bust voters who are too blinded by ideology to see the self-evident truth and data behind Clinton's perfectly rational platform," or else some nebulous, nefarious Russian conspiracy to deny our supremely qualified candidate from victory for equally vague and nebulous Reasons (tm). It's the same driving ideology that manifested in the Pied Piper Strategy, in which Hillary literally conspired to advance the candidacy of Donald Trump above other Republicans (because against any other Republican candidate it becomes that selfsame battle of qualifications and 'correct data-driven conclusions' whereas against Trump it's a battle between a supremely qualified candidate and a supremely unqualified candidate, and the winner of that should be self-evident). It's the same reason why Bernie and his supporters are treated on MSNBC and CNN alternatively, either as well-meaning but misguided Liberals who need only to be shown the data so they'll realize the absurdity of their chosen candidate, or, failing that, cajoled or shamed into giving up their candidate in the interest of "party solidarity," or, failing that, suppressed and destroyed because they are ideologically-, rather than data-, driven and that makes them equivalent to Trump - an existential threat to our democracy and the formulation of rational policies.
To conclude, I will quote Gottfried Leibniz, epitomizing the optimism of the Enlightenment project, positing a supremely rational, positivist future in which:
"[...] if controversies were to arise, there would be no more need of disputation between two philosophers than between two calculators. For it would suffice for them to take their pencils in their hands and to sit down at the abacus, and say to each other (and if they so wish also to a friend called to help): Let us calculate."
That is the modern, post-Clinton Democratic party, neatly summarized.