Colorado GoP already publicly said if Trump is not allowed on the ballot, they're changing from a primary to a caucus to get him back on. So, regardless of what happens, Trump will be a candidate in the primary.
That leaves the other candidates with two options:
Side with Trump against the Democrats' insane manipulation attempts or
Not side with Trump against the Democrats' insane manipulation attempts
Colorado GoP already publicly said if Trump is not allowed on the ballot, they're changing from a primary to a caucus to get him back on. So, regardless of what happens, Trump will be a candidate in the primary.
That leaves the other candidates with two options:
DeSantis chose option 2.