Because a restriction on code contribution makes sense due to the fact there's often only one area where the code makes sense. If it doesn't fit the vision of whatever the code is trying to do, then there's no reason not to restrict it.

Financial contributions affect all areas of the organization and the needs/wants of the organization changes daily, some times spontaneously. People simply donating to "writing code" doesn't make an organization function. There has to be some sort of PR. There has to be some sort of administration. There has to be marketing, quality, distribution, etc.