Because tax exempt money can’t be used for political purposes.
But you get to claim a tax deduction when you donate to political parties.
‘The tax law is relatively clear about what a (c)(4) can and cannot do. The IRS defines these groups as “civic leagues, social welfare organizations, and local associations of employees.” Their net earnings are supposed to be used for charitable, educational, or recreational purposes. They may lobby and participate in political activities but their primary purpose must not be campaigning.’
So they can participate in political activities. Maybe through fundraisers or whatever. But they were denied the chance, while Dem organizations were approved, and I do believe it affected the election.