Most startups do not want to make immediate shareholders of new people as the company is growing. They want to test them first. Hence, the standard grant of options with one-year cliff.
Another important reason not to use restricted stock as the norm: you have to pay for the stock up front and (once you get beyond the founder pricing which normally is not offered to other early-stage people) this creates financial risks for employees who would rather not commit cash to the company until they see how it plays out (with options, you need pay only when you decide to exercise). tptacek makes this point well elsewhere in this thread.
This practice of limiting restricted grants to the founder group only is not legally required but is widely done as a prudential matter. I think it is a strongly held belief among founders and investors that it is unwise to be too loose in making people shareholders. Thus, getting a restricted-stock grant has, by custom and habit, become a sort of "founder's privilege."
We have had companies that use restricted stock freely beyond the founder group. In the right cases, this works fine and sometimes there are even special reasons why it is critical to use this approach (e.g., after a small equity round, to avoid 409A tax problems where the company wants to make grants at an aggressively low price but doesn't want to pay for an outside, independent appraisal to keep the grants within the 409A safe-harbor provisions - in such a case, using options can be dangerous while using restricted stock can avoid the tax problems).