What is the point of inverting RAM output?
I also don't know for sure (those who know for sure are likely dead now.) But I'll take a shot.

The 7489 was an early TTL memory device. Tri-state output wasn't available. Since it was also a big advantage to be able to wire-AND without additional output buffering, open-collector outputs was the obvious choice for devices that might have to share a bus.

As you probably know, NPN BJTs were used almost exclusively in those early circuits. (Just look at the schematic for a TTL logic gate.) NPN BJTs are pretty good at sinking current and their open-collector outputs could sink up to perhaps \$16\:\text{mA}\$ with the 7489.

One of the interests, at the time, was to directly hook up LEDs to the outputs. There was sufficient sinking capability to drive an LED. But.. well, to do that you needed the open-collector to be active. So if you directly hooked up your LEDs (and current limit resistors) to the outputs, then you would see lit LEDs where "1"s were written. And folks liked that feature. So I think the designers of these early memory chips chose, in part, to make the outputs inverted to help support the simpler use of LEDs to represent the bits.

Another reason is that these outputs would be wire-ANDed with a pull-up resistor. Before that output could be used again, it needed to be buffered. This buffering was almost always done with a 7404 inverter. Which inverted the signal! (And so you got your "1" back.)

When the 74189 came out, I suspect there was a desire to offer tri-state outputs but to also allow it to be used where the 7489 was previously used (tri-state or active-LO mode only, for example.) So they kept the behavior but added tri-stating to it. (I recall that they later put out the 74219, which doesn't invert.)

Those issues certainly motivated me when I worked with the 7489 and I think that's the reason that the 74189 originally chose to stay with the earlier design concept. Of course, as things proceeded in time, more became available and for less money and the motivations evolved.