That they, the Christians, caused the fire to burn a large part of the Roman Empire down.
The most likely reason for women to become heads of state is the general progress in gender equality, which slowly introduced women to position of moderate power (parliament), which prepared ground for their position as a head of state.
The two word term would be Power vacuum.
Explanation:
A power vacuum is what happens when there is sudden loss of authority in the governance and there is no potent replacement of it in sight.
This is usually what happens after the death of a long reigning dictator or a strong leader who had held the country so that there was no other alternative ready.
So the stability of their governance leads no potent alternative and makes a power vacuum in the nation.
The power vacuum will be the void of no strong leadership here.
