why is the breast cancer heving the sign of th number 8 without the its lower margin?

Answer 1

It's not an "8", it's a ribbon. Ribbons have been used for years to symbolize support for various causes. Pink is for breast cancer. Yellow is for the safe return of the troops. Other colored ribbons symbolize other causes.
It is a visible way to show support, by wearing on a lapel or other clothing, usually worn close to the heart.

Answer 2

Actually it and all symbols like it are representations of a ribbon that has been curled back on to itself and pinned to hold it together.