The Cube was really aimed at the academic research market, and it sort of did ok there as far as sales went. As a research system it had some audio and DSP features and it was up to users to write their own code to do something interesting with them.
The Fairlight was aimed at the high end music market. It did some very specific things and nothing but those very specific things. It was super-useful for music production, but it was - in its high-end way - a closed end user product, not an open development system.
The problem with the Cube - aside from the price - was that the DSP hardware was still quite slow. Although you could develop your own applications they were very limited compared to the commercial synths, samplers, and FX processors that were available commercially.
So it was only really useful for prototyping new concepts for applications, rather than producing finished applications.
That's where Max came from. Initially it was a very minimal sort-of-modular MIDI processor. The IRCAM merged it with their Cube-based synthesis and processing system. Eventually that became Max/MSP, a commercial product with both MIDI and audio generation and processing. (Then video and low-level custom DSP were added, but those are much more recent.)
