They were extremely limited in the resources they could bring to bear on the genome. Sequencing was expensive and complicated, so even by sequencing several pe

They were extremely limited in the resources they could bring to bear on the genome. Sequencing was expensive and complicated, so even by sequencing several people the best they could do was make one composite image of the whole genome. Representing things as a graph would have provided no benefit at the time, and so people didn't do it. Now we have thousands of public sequenced genomes. We need a new model for how we manage genomes. We are also learning how much is missed in the standard linear model of the genome, and need a way to incorporate new information into the reference. This all takes time. Graphs are a good ways of encoding using our prior knowledge of genomes, but they are also difficult for researchers who have grown up on linear systems to understand.