1) Mendel's work "appeared" after Darwin's. The Origin of Species was published in 1859. Mendel presented his work on 1865. True, Mendel did send a letter to Darwin explaining his work but it seems very unlikely Darwin ever read or understood it. Mendel had a very obscure and hard to understand writing style.
2) By "appeared" above I refer to the fact that Mendel's work remained completely ignored for decades. All of his scientific papers were burned by the abbot that succeeded him in the monastery. It was Hugo DeVries that rediscovered his work at the beginning of 20th century. Again, Mendel's hard writing style played against him.
3) Mendel never cared about random mutation and natural selection, the fundamental elements of evolution. He only recorded the combinatorial patterns on pea's traits deducing the recessive/dominant traits. Truth be told, today these are a small part of gene expression. The metabolic pathways of gene expression are much more complex than just the recessive/dominant pattern.
