Mendel was very very lucky that he chose the traits that he did. They happened to be single gene traits. If he had picked any number of multigene traits, it would be very difficult if not impossible to work out how simple gene inheritance works.
EDIT: To clarify, I suspect he already had observed that there was a pattern present and based his hypothesis on that before doing any real testing to prove it.
Mendel was very very lucky that he chose the traits that he did. They happened to be single gene traits. If he had picked any number of multigene traits, it would be very difficult if not impossible to work out how simple gene inheritance works.
Lucky, or observant?
EDIT: To clarify, I suspect he already had observed that there was a pattern present and based his hypothesis on that before doing any real testing to prove it.
https://scienceprimer.com/punnett-square-calculator
This is what a 5 gene trait looks like: