Mendel’s work was unrecognized till 1900 A.D. because 
I. In those days, communication was not easy 
II. Genes that control the expression of traits as stable, was unacceptable because of continuous variations in nature.
III. Mendel's approach of using mathematics to explain biological phenomenon was not accepted by biologists of his time.
IV. Mendel did not provide any chemical or physical proof for the existence of unit factors. 

Mendel published his work on inheritance of characters in 1865. But for several reasons, it remained unrecognised till 1900. Firstly, communication was not easy (as it is now) in those days and his work could not be widely publicised. Secondly, his concept of genes (or factors, in Mendel’s words) as stable and discrete units that controlled the expression of traits and, of the pair of alleles which did not ‘blend’ with each other, was not accepted by his contemporaries as an explanation for the apparently continuous variation seen in nature. Thirdly, Mendel’s approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time.