I should unpack that a little more. The fundamental Unix conventions were established in the 1960s and 1970s, when computers were far more limited in CPU, memory, and network bandwidth.  Every keystroke counted; being able to type 'ls -slApF' instead of 'ls -s -l -A -p -F' was a significant savings in time and resources (nevermind 'ls --size -l --almost-all --indicator-style=slash --classify', which wasn't even an option in the early days!). Was it hard to read?  Sure, but it worked!  Cheat sheets were common, but any command one used frequently became second nature, plus 'alias' let you encapsulate a longer command in a short string.

Over time, as bandwidth increased and other resources became more plentiful, readability and memorability became bigger concerns, and some commands started running out of single letters that were in use, especially letters that had any connection to the parameter.  But breaking everything that used the original format -- including user habits -- would be a really bad idea, so the -- predicate for multi-character parameters was introduced.  The most commonly used options still have short names for efficiency; less-common options may only have a long name, and many options have both, because there's more than one way to do it, as Perl programmers say.

I'm oversimplifying a bit -- other conventions have been used and sometimes still are; some tools use a - for all parameters and don't allow combining, some use +/- to turn options on/off, etc. -- but that's the general idea.

'/' is a directory separator in Unix, so it would not work well as a parameter predicate.