Dartmouth BASIC was designed for teletype-style, hard-copy printing terminals, rather than video displays. Conveniently your whole session was printed out, so you could take your email and program listing home with you. Line-by line editing was practical for printing terminals, and line-by-line I/O scaled well across multiple terminals on a timesharing system.
Line editing also worked well on microcomputers with cursor movement (like the C64) - you could edit code in place just by overtyping and hitting "return" for the appropriate line.
On a slightly unrelated note, teletypes date back to the 19th century telegraph (and typewriter) era.
And still exist and are heavily used today, RTTY is built into most HF radios, and most militaries still use radio teletype, but encrypted and often at a different bandwidth.
Line editing also worked well on microcomputers with cursor movement (like the C64) - you could edit code in place just by overtyping and hitting "return" for the appropriate line.
On a slightly unrelated note, teletypes date back to the 19th century telegraph (and typewriter) era.
https://en.wikipedia.org/wiki/Teleprinter