The reason capital gains are only partly taxed is because part of the apparent gain is the result of inflation and, therefore, no real gain at all. Capital gains were once taxed at 75 %, but this was reduced to 50 %. With inflation as low as it is now, bringing the rate back to 75 % doesn't seem unreasonable.

The reason for the dividend tax credit is that the corporation has already paid income tax. If the shareholders had to pay income tax on the full amount of the dividend, there would be double taxation. The credit is supposed to offset the tax that has already been paid. The foreign tax credit is based on the same principle.

The logic is that "a buck is a buck is a buck" (for the history behind this, see https://www.policyalternatives.ca/publications/commentary/putting-fairness-back-canada%E2%80%99s-tax-system ) and should be taxed at the same rate. It isn't based on the idea that any particular group should be rewarded.