Bierko had been making the rounds since the mid 80's. His biggest claim to being box office, IMHO, was his stint on SEX AND THE CITY. I heard (somewhere) that this particular director really wanted him for Harold Hill. One can only wonder why.
I have to assume Stroman was hellbent on having a younger and notable sexy Harold Hill (in a way Preston just was not, not typically anyway), and wanted the title and show to be the star, not the names of the two leads.