Why wasn't Fermats last theorem written on paper?

But it was. That is why we know about it. If you mean why the PROOF was not written- Fermat wrote that he had found a wonderful proof for the theorem, but unfortunately the margin was too small to contain it.

This is why the theorem became so famous- being understandable by even a schoolchild, but at the same time so hard to prove that even the best mathematicians had to surrender, with a simple proof seemingly being existent that just nobody except Fermat could find.

The theorem has since been proven but the proof uses math tools that are very advanced and were not available in Fermat's life-time.