Because policies are more of a matter of "how" rather than of "why". For a policy to be even worthy of the name, it necessarily has to assume the status quo as a starting point and then build up from that. To limit a discussion to strictly policies (rather than politics) is to confine the end result to a state only a few legal projects away from the status quo. And the status quo varies from country to country, so this discussion format doesn't favor international participants.

Consider a change as complex and as sweeping as the Revolution of 1917. It's something that stays within the realm of relevant political discussion, since, well, it happened and it deeply influenced the history of many countries, yet how could it imaginably qualify as a policy discussion?