My speculation is that washability/longevity comes down to spun fiber poly (most bulk) vs. multi continuous filament poly (trilobal).
Spun fiber combines many short fibers into a longer threads. The zillions of fiber ends work their way loose with use, weakening the string, shedding lint, making the string more fuzzy and making it more prone to grabbing itself in ways that alter performance. Washing out dirt and oils may provide some benefit but probably accelerates the fiber degradation.
Continuous filaments run the entire length of the string. Individual filaments in a string can break, but the scale of the degradation is smaller and slower. This means that the string performance is more consistent over time. These strings can be washed without reducing longevity.
The longevity-reducing properties of spun poly also allow more elasticity, softer strings and strings with more variability in other properties (within a certain window of usage). Any of these could be a priority over string works consistently over months or years.