According to Systems and methods for a self-deploying vehicle drag device (US 8616496 B2) patent's background of the invention:
The United States Federal Communications Commission (FCC) requires CubeSats and other picosatellites to be designed to re-enter the atmosphere within 25 years of the end of their useful lifetimes. Without an assistance, it is estimated that a CubeSat may take over 150 years to de-orbit from an 800 km altitude.
The purpose of deorbiting devices is to cause the satellite to deorbit within the 25-year post-mission lifetime limit, as also recommended by the Inter-Agency Space Debris Coordination Committee (IADC) and ISO 24113 Debris mitigation requirements and compliance:
(e) The orbital lifetime of objects passing through LEO (lower than 2,000 km) shall be shorter than 25 years after the end of operation.
Why does it take 150 years for a CubeSat satellite in 800 km Low Earth Orbit (LEO) to de-orbit naturally, without the use of deorbiting devices?