Bluebird Bio Inc. CEO Nick Leschly conceded that it’s “hard for folks on the outside looking in” to understand why the firm would cleave its severe genetic disease (SGD) and oncology efforts into two independently traded public companies, but said the Cambridge, Mass.-based firm is making the change by the end of this year in order “to make sure we can basically fall down and get back up and learn everything we can in the most disciplined manner,” he said.