Why is paying someone to murder someone a different category than other forms of speech?
Why are intentionally deceptive drug advertisements a different category than other forms of speech?
Why is pointing a gun at someone and telling them to do something a different category than other types of speech?
I'm going to claim that it's because these forms of speech enable types of coordination and coercion that corrupt processes and structures that we deem essential for the operation of society. So, pragmatic reasons.
Placing limits on the payments, direct or indirect (a dollar I spend is a dollar a candidate doesn't have to spend), that can be made to aid people in gaining a position that places them in charge of allocating public resources, that's something that very few people seriously argue against. What they argue about is what those limits should be.
You can make an argument that payments to candidates and government officials should be allowed, but making absolutist arguments that something is speech and therefore must be permitted isn't serious. Payments to a television station to run an ad that you financed the production of is surely less worthy of being called speech than calling in a bomb threat is - at least the bomb threat is using your mouth to say a thing. Speech, instead, is what we define to be speech. If it's to encompass all expression, you might as well call selling cigarettes without a tax stamp speech.