Mendel’s work remain unrecognized till 1900 because
(a) Communication was easy
(b) His mathematical approach was new and unacceptable
(c) The concept of genes (factors) as stable and discrete units could not accepted by others
(d) He could not give physical proof for existence of 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.

 Finally, though Mendel’s work suggested that factors (genes) were discrete units, he could not provide any physical proof for the existence of factors or say what they were made of.