❓ why Mendel proposed the idea of separately inherited factors that
we now call genes;

❗ Sorry, it is too simple.
Mandel proposed the idea because the ratio of beans implied the idea of separately inherited factors. he classified the characteristics of beans and recognized that there is a certain ratio. So he knew that the ratio can be explained by the idea. This was very revolutionary because he related biology with math. As far as i know, this was the first attempt to relate them.

❓ why the importance of Mendels discovery was not recognised
until after his death.

❗ Because Mandel was not a professional biologist but you know, he was a monk. And he was the first one who related math with biology, most of biologists had an antipathy with his theory. Mandel sent a letter about his theory to Darwin, but this letter was discovered in one of the books Darwin had : He probably didn’t read it!
Anyway, because of his obscurity, his theory was ignored : I think it is a tragedy.