Why rush to become multiplanetary?

“…that window of opportunity may be open for a long time or it may be open for a short time but we should not assume that it is open for a long time we should take action and become a multi planet civilization as soon as possible…”

— Elon Musk, 17 September 2018 · Watch: SpaceX BFR Lunar Mission Announcement · 26:53 ↗