The designers, I suspect, looked at the other components of Canada's domestic law provincially, so marrying within Canada would be subject to those provincial laws. Those provincial laws have floor limits on the age of marriage; hence, it would not necessarily be a good idea to have, federally, a minimum marriage age. Having that would appear to be an intrusion into provincial jurisdiction.
It wouldn't be necessary to enforce the intent of Parliament as proposed in the bill by having a minimum marriage age when the minimum marriage age is evident in provincial statutes.