They didn’t force anyone to do anything, they bullied them out of it.

They cancelled primaries in Florida and North Carolina entirely, which took those delegates off the table for any challengers. They changed primary dates for 2024 which made it harder for other candidates to get on the primary ballots, and pledged to reduce the number of delegates that a state received if they held their primary on their original schedule instead.

Last year, the Rules and Bylaws Committee voted to strengthen their penalty power over states that jump the line. Not only will those states automatically lose half their delegates, the DNC also broadly empowered the national party chair to take any other “appropriate steps” to enforce the early window.

It also nearly eliminates any path for a potential Democratic primary challenge ahead of 2024 by elevating states that represent the president’s base of support.

The rules change in particular was initiated in 2022, which given it’s placement of Biden-friendly states at the start (to generate immediate momentum in delegates) was about as close to explicitly telling other democrats not to run as if they’d just sent out a newsletter.

Lastly

Bernie didn’t get nominated because not enough people showed up to vote at the end of the day.

If this is in reference to 2016, it’s more because he was robbed of it by the Hillary campaign controlling the DNC and using them to disadvantage him at every turn. But don’t take my word for that, take Donna Brazile’s, the head of the DNC at the time of the election.