"Because she discovered DNA and was basically screwed over by the scientific establishment when it then gave credit to two men - James Watson and Francis Crick – who basically confirmed her findings."
DNA was first isolated by Friedrich Miescher in around 1869, and by the beginning of the 20th century Albrecht Kossel, who got a Nobel for his trouble, had characterised the nucleotides that are the key components of DNA and RNA. Franklin's work was of course crucial for the discovery of the _structure_ of DNA, but she did not arrive at Watson's critical insight (informed by Erwin Chargaff's work on nucleotide ratios in DNA, discussions with colleague Jerry Donohue, and published work by June Broomhead and others) - that Adenine pairs specifically with Thymine, while Guanine pairs with Cytosine, on the inside of the double helix. Franklin should receive full recognition for the importance of her work without over-simplifying or distorting the events that led to the DNA structure. It's tragic that she was not honoured for it in her lifetime, as she may have been in one of the 1962 Nobels if she had lived.