Important parts of Unix's history happened before readline support was common
Unix and things that run on Unix have been around for a long time now. In particular, GNU Readline was first released in 1989 (as was Bash), which is long enough ago for it (or lookalikes) to become pretty much pervasive, especially in Unix shells. Today it's easy to think of readline support as something that's always been there. But of course this isn't the case. Unix in its modern form dates from V7 in 1979 and 4.2 BSD in 1983, so a lot of Unix was developed before readline and was to some degree shaped by the lack of it.
(This isn't to say that GNU Readline and Bash were the first sources of readline style editing, command completion, and so on; on Unix they go back at least as far as 1983, with tcsh. But tcsh wasn't pervasive for various reasons.)
One obvious thing that was shaped by the lack of readline was csh. Csh has a sophisticated set of operations on your command history that are involved through special strings embedded in your command line. To quote the 4.2 BSD csh manpage:
History substitutions place words from previous command input as portions of new commands, making it easy to repeat commands, repeat arguments of a previous command in the current command, or fix spelling mistakes in the previous command with little typing and a high degree of confidence. History substitutions begin with the character `!' and may begin anywhere in the input stream (with the proviso that they do not nest).
The most well known history substitution for tcsh users is probably
!!', which repeats the previous command. Bash has a similar
and even today the Bash manual calls out its similarity to csh's
version. These days I suspect most people using Bash don't use Bash's
history substitutions and just stick to readline stuff; it's generally
more fluid and easy to deal with.
(This is an obvious observation, but at times it's easy to blur the old days of Unix together and lose track of how comparatively old some parts of it are. Or at least it is for me.)
PS: My impression is that the widespread availability of command and filename completion subtly shapes the kind of command names and file names that people use. When you don't have completion, it makes a lot of sense for names to be short and it doesn't matter if they're all jumbled together so that completion can't tell them apart. Famously, Unix loves short command names because they're short to type, which makes a lot of sense in a V7 environment.