Because that's what the logo looked like:

https://commons.wikimedia.org/wiki/File:Smithsonian_National...

It's kind of a fun shibboleth for those of us that hacked on the systems back in the day. The ROM used ][ instead of II and we used that string when we discussed the computer online. I still type it that way just out of pride.

It's kind of like C64 hackers that say KERNAL instead of "kernel" since that's what Commodore docs called it.

And for added fun, although ] and [ were in the ASCII subset that the Apple ][ used, they were not on the keyboard. I remember writing a program in AppleSoft Basic that was a very limited word processor that used the ctrl key as shift and displayed uppercase using inverse (which was a bad choice because then the return key gave M as its output rather than a new line), and finding that I could get half the logo through the esc key.

> those of us that hacked on the systems back in the day.

Thanks for letting us stand on your shoulders :)

And followed by the Apple ][ plus

I think you could purchase a kit with a replacement ROM and case face plate to convert the Apple ][ to an Apple ][ plus

Edit Add;

Integer BASIC : https://en.m.wikipedia.org/wiki/Integer_BASIC

Applesoft BASIC : https://en.m.wikipedia.org/wiki/Applesoft_BASIC

Officially Woz : http://www.woz.org/letters/apple-basic/

...and followed by the //e which only used that string on boot-up if you had the enhanced chipset installed.

It was just a popular stylization of the roman numeral that stood out, and more accurately reflected the logo on the actual device: https://en.wikipedia.org/wiki/File:Apple_II_typical_configur...

][ also appears first in the boot sequence

At some point it changed to // as in: Apple //e on the boot screen and the case.

The front plate of the Apple 2 had it designed like "][", a more compact "II" I guess. Look for Apple 2 logo pictures online for an idea of what it looked like.