Hi-Fi TV Soundtracks (pre 1978) - Why?

[kfbkfb]

I was listening to the closing theme music of Voyage to the Bottom of the Sea
on MeTV last weekend on my system (Pioneer VSX-D209 + 4 Polk T15s).

IIRC, in 1978, TV sound distribution (USA) was upgraded.

The VTTBOTS soundtrack had amazing fidelity, with musical instruments
whose sounds wouldn't survive the (pre 1978) TV sound distribution
(except in the cities where the Network signal originated - NYC/LA etc.).

I'm wondering why composers created TV soundtrack music using
musical sounds that wouldn't be heard by the majority of TV listeners.

Kirk Bayne