why didn't mendel notice the chromosomes?

Answer 1

Because electron microscopes were not available at the time. Also, he was studying the effects of heredity rather than the mechanism by which hereditary material was passed.

Answer 2

The discovery of the double helix required of James Watson and Francis Crick an enormous amount of hard work using the very latest technology, and no small amount of luck in the guise of the x-ray photographs taken by Rosalind Franklin.

This significant accomplishment, which won the discoverers the Nobel prize almost a century after Mendel's research, was simply not possible with the technology available in Mendel's era.