I can only find references to it in other articles but apparently Famitsu 0220 ran a feature where they consulted with several STG manufacturers & other people on why horizontal STG default to left-to-right scrolling rather than right-to-left—here's what the devs said: (thread)

Capcom: just as vert STG default to the ship at the bottom facing upward, hori STG default to the ship on the left scrolling rightward so as to align with the placement of the stick & better synergise with the player

why stick position defaults to left side, they don't know

Namco: stage data is naturally written from top-left to bottom-right so games tend to scroll left-to-right without giving it much thought

Namco's Sky Kid scrolls right-to-left in consideration of JP players but they didn't get much feedback either way & surmise either's fine

Konami: basically, human nature tends to default to left-to-right for a great many things so left-to-right scrolling is the most natural pick

whichever PR rep they talked to is like "even if someone makes a right-to-left game, I don't want to play it, it'd make me queasy"

Irem: so much other media has stuff emerging from the right that it you want to focus& direct ppl's attention to the action, left-to-right is the obvious option

R-Type's st.3 scrolls leftward in spots but again, decisions about scrolling direction weren't given a ton of thought

the article also surmises that the default "stick on the left, buttons on the right" arcade setup was cemented by Space Invaders—specifically, the cheaper cabinets with the stick on the left that operators would recycle with different boards