Because he didn’t register to be on the ballot since NH threw a hissy fit over not being directly after the Iowa caucus as it has been since forever. So the DNC removed their delegates for choosing a presidential candidate and now the NH primary has no baring in choosing the democratic candidate for president.

From the article: (emphasis mine)

Last year, Biden tried to end New Hampshire’s 100-year reign as the nation’s first presidential primary state when he directed the Democratic National Committee to overhaul the 2024 primary calendar by putting South Carolina (which he won in 2020) ahead of New Hampshire (which he lost).

New Hampshire, whose political class holds the primary as sacred as the Red Sox, just went ahead and scheduled its primary in its usual place right after Iowa’s caucuses, essentially telling the DNC to go pound granite and even threatening the committee with legal action. (As a result, the DNC says it won’t seat New Hampshire’s delegates at the summer nominating convention.)