His experimental results were too good to be true.
https://statmodeling.stat.columbia.edu/2012/08/08/gregor-mendels-suspicious-data/
Or perhaps his data was different than some would expect because he was working with biological characteristics.
Also, Mendel sent his paper out to the top scientists of his time to be critiqued.
“it should be kept in mind that Mendel not only anticipated but also would have welcomed repetitions of his experiments by others. He would not have benefited scientifically, financially, or ecclesiastically from any outright misrepresentation in his work.”
https://academic.oup.com/genetics/article/166/3/1133/6050294