Canada lists ordinary shares instead of ADRs on NYSE.

Looking at the foreign companies that dual list on American exchanges like NYSE, it seems like some list ordinary shares and some use ADR’s. Canadian companies, for example, seems to only list ordinary shares on NYSE not ADR’s. Is there a specific reason for this? Why do some countries seem to list all ordinary, all ADRs, or mixed? Also. why would a company choose one over the other? Thanks