Ray Hoad wrote:

Thus weekly updates are completely adequate with one exception. The ISS is an example of an object that has the ability to change its orbit. Thus the TLEs for the ISS are updated more frequently.

Another motivation for frequent TLE updates for ISS is the narrow beamwidths of dish antennas used for 2395 MHz HamTV. Even without orbital maneuvers ISS TLE accuracy can degrade enough to cause antenna pointing problems within a week.

de KM1P Joe