Democrats are debating this because of the delay in 2020's caucus and some believe Iowa does not represent the entire US.
Before the election, several people from a party choose to run for the election and then these candidates participate in primaries. The first event of the primary season isn't a primary at all - it's a series of caucuses in Iowa. BBC notes that "A caucus involves people attending a meeting - maybe for a few hours - before they vote on their preferred candidate, perhaps via a head count or a show of hands. Those meetings might be in just a few select locations - you can't just turn up at a polling station. If any candidate gets under 15% of the vote in any caucus, their supporters then get to pick a second choice from among the candidates who did get more than 15%, or they can just choose to sit out the second vote".
Iowa's first position became a topic of debate after their disastrous caucus in 2020. Results were delayed for days due in part to a faulty smartphone app. Eventually, The Associated Press never declared a winner in the contest because of problems with the vote count, which was administered by the Iowa Democratic Party. Although, an audit into what went wrong on caucus night pinned the blame on the DNC (Democratic National Convention), which made last-minute demands of the app's developer.
NPR notes that "Iowa's voters are also older, more rural, and more white than many other states, so it's seen as increasingly out of step with the Democratic mainstream, which increasingly relies on voters of color and young people for its support".
Vox notes that "Under the current DNC rules, no state except the first four — Iowa, New Hampshire, Nevada, and South Carolina — is allowed to hold its primary until after “the first Tuesday in March.” The early statuses of those four states, meanwhile, are explicitly protected in the rules".
DNC member Clay Middleton said that there might be a lengthy debate about the nominating calendar. Wendy Davis, a DNC member from Georgia, said that it would be a good idea to have all the early states go closer together. The party has not decided on the issue, and the White House is expected to take a final call on this. However, state party leaders have remained committed to the notion that Iowa should go first.