Why is hexa-decimal no system is used instead of octal no system then working with 8?

Well, when working with 3-bit values, then Octal is used. In some early systems with 6-bit 'bytes', octal was used a lot.

But fairly rapidly, 8-bits was chosen for the base unit, as it matches better with binary, as 8 is a power of two. And Hex fits 8-bit values like a glove: 2 hex digits match 8 binary digits.

00 00000000

01 00000001

08 00001000

0f 00001111

ff 11111111

Today, we use 32 or 46 bit 'Words', and these match perfectly with hex digits. Octal would just be a mess, so it has largely passed into history, waiting to catch any unwary new C programmers who use leading zeros.