Scott Manley answers this question here.
Basically, it is due to the "shelf life" of the hydrogen peroxide propellant used by the attitude control thrusters of the Soyuz descent module. When hydrogen peroxide is passed over a heated catalyst, it decomposes to water and oxygen, and releases lots of energy, so the propellant goes from liquid to steam/gas, providing the thrust. But hydrogen peroxide will also slowly break down spontaneously, so the stored propellant will gradually lose its potency.
The 200 day value is a threshold of safety: decomposition hasn't progressed too far to affect usability of the peroxide propellant to perform a controlled re-entry, landing within a targeted area.
He also goes on to describe the consequences of an uncontrolled (ballistic) re-entry - not a disaster, but definitely undesirable because of G forces imposed on the crew and unpredictability of the actual landing point.