Subotnick's recordings first came out in 1967. The problem you have noted with instruments being panned hard-left or hard-right was very common on many recordings in those days.
In the mid-1960s, most record buyers had monaural (mono) "high-fidelity" or "hi-fi" sound systems and turntables and reel-to-reel tape machines that could only play back monaural (mono) recordings, not stereo.
When stereo systems appeared on the market, record labels wanted to sell the new stereo phonographic disks, and the public was eager to buy them.
At this time radio was also transitioning from mono (on the AM band) to stereo (on the FM band) and people were buying the first stereo radio receivers for their homes and cars.
The problem is that while recording studios would record the music on multi-track tape (at this time on 3 or 4 tracks, or in rare cases 8 tracks) the recording engineers only had experience in mixing down to monaural, and they would deliver the final album masters as a monaural recording. Recording engineers were just beginning to work with stereo equipment, and they simply had not established any standard methods for mixing stereo sound. Panning instruments or singers' voices hard-left or hard-right was something you find in a great deal of all kinds of recordings from this transitional era, apparently because the mixing engineers had not thought of a better way to do it.
The most famous example is the albums of the Beatles. The albums, starting circa 1963, were recorded on a small number of tracks that were mixed down to mono. Around 1965, the Beatles' record label asked the Beatles' producer George Martin to make additional re-mixes of the Beatles in stereo. The same Beatles albums could be bought in a mono phonograph or stereo phonograph version.
The stereo re-mixes of the Beatles are notoriously hard to listen to over headphones. All the guitars might be panned hard-right while all the vocals are panned hard left. Or Paul McCartney's lead vocal is panned hard right while all the harmony vocals are panned hard left, for example. History says that at first the producer George Martin did not want to involve himself with stereo, so he supervised the original mono mixes but left the stereo remixes to his employees and did not give much attention to the process.
The Beatles did not record a new album that they intended to be listened to in stereo until the Abbey Road album in 1969.
The primitive stereo re-mix versions of the earlier Beatles albums were the only ones available for people to hear from the 1970s all the way up until 2009 when a multi-CD box-set collection called The Beatles in Mono was released.
Since I bought the Beatles in Mono boxed set, I'm like you: I can't stand to listen to the Beatles' primitive stereo mixes anymore, even though the stereo mixes were the only ones I heard my whole life, for about 40 years, until the mono set was released.