Probably many reasons. One reason is that bar codes only need to represent digits 0 to 9. ASCII can represent 127 different codes. If you used ASCII you'd be wasting an awful lot of bits.

I guess in bar codes too you don't want more than 3 or 4 0s or 1s in a row.

I know and understand ASCII and today I was looking at how barcode's work and I saw that they get 1's and 0's from the black and white lines on the barcode, witch was OK for me but i still don't understand how the binary code they use is different from the binary code computers use.

For example I know that:

4 in barcode: 1011100

4 in ASCII:00110100

I just don't get it. How is it not the same, does it use a different code or something?