I can’t find the reference now, but as I recall* it came up a couple of years back and someone at FBG said that it’s basically a real-world computational constraint. Larger acquaintance lists cause problems because they become rapidly more demanding (ie allowing each list to have twice as many entries causes it to consume more than twice the resources).
Since most players don’t need to be in routine contact with 100+ other players, the current system doesn’t really limit most of us, and so there are always higher priorities for people to work on.
*but human memory, time, etc. I could be misremembering something completely different.