probably to eliminate poor fuel economy from parasitic losses?

Twin-charging was just something that several Japanese manufactures played with.
The supercharger negates turbo lag at lower rpm, and the turbo utilizes wasted energy to provide boost, and relieve supercharger power draw at higher rpm.
Toyota also did it on the GX81 Chaser sedan.
And HKS made a kit for the 4AG for twin-charging it.

Both have to be sized correctly for the engine they are going on, and both would make boost by them selves, without the other,
but over all performance would not be as good.

In the end, nobody does it any more. I guess because of the cost and complexity of the systems.

Nissan only built 500 of the super-turbo March cars per year.
I believe it was just to meet production requirements of a class of racing there in Japan.