If Democrats are truly fractured, why is Biden doing so much better than Trump in primaries?

[Respect and Compassion]

The thread poses a false premise. Biden isn't doing "so much better than Trump" in primaries so far. In fact, there's a case to be made that Trump's performance is stronger despite the overall lower percentages aside from the Nevada caucus.

There has only been one (1) primary with both Trump and Biden on the ballot: New Hampshire. The premise of this thread behaves as if Super Tuesday already passed (or if we're even further ahead in the 2024 primary cycle) and Biden outperformed Trump's percentages and vote totals (scaled by partisanship in each state) by a lot.

In New Hampshire, there was a push from Nikki Haley and Chris Sununu to get anti-Trump who would vote for Biden over Nikki in the general, to vote for Nikki in the Republican primary. There was no such strong push from prominent figures to get Trump supporters to vote for Dean Phillips or Marianne Williamson or some other non-Biden candidate in the Democratic primary. Also note that Trump's raw vote total well surpasses Biden's write-ins, and that Biden well underperformed Trump's 2020 percentage in Trump's uncontested NH primary.

Trump wasn't on the ballot in the Nevada primary, but his percentages in the Nevada caucus are sky high, comparable to Biden's percentage in the South Carolina primary. Moreover, Trump's vote total in the uncompetitive 2024 Nevada caucus is so high that it's not far from the combined totals from Trump, Rubio, and Cruz in the competitive 2016 Nevada Republican caucus. In comparison, Biden's 2024 primary vote total in Nevada seems like it won't even exceed Catherine Cortez Masto's 2022 primary vote total.

Biden had no Iowa Caucus equivalent where multiple prominent Democratic figures spent well over $100,000,000 combined to carve out caucus support for them. Imagine if Gavin Newsom and Gretchen Whitmer spent that much in a hypothetical 2024 Iowa Democratic caucus.

Also note that overall turnout in the Nevada Democratic primary and the South Carolina Democratic primary was rather lacking, indicating Biden will have turnout issues with hispanics and black voters. Trump by comparison well improved on his 2016 caucus performance in Nevada, and also well improved in his 2016 NH primary performance in terms of both percentage and raw vote totals.