Here's a nice timely example of why a party would be more interested in subsidies based on fundraising than votes.

Let's say you're a developer and insurance guy behind a group that wants to bring private healthcare to Canadians - something Canadians have said they will not vote for. Sure you'll donate to the limits to your local candidate whose party is most ideologically inclined to do what you want. You'll likely yowl to your fat cat friends about how unfair your tax deduction is compared to what granny gets for her $20. But what do you really care about your tax deduction, when your investment of a few grand brings you a $10 million cheque?