It's because companies can own companies. You can have one company own another company in another country, which owns another company in another country. The ability to track where assets are actually owned, and who they're owned by, becomes incredibly opaque, because you're dealing with different legal systems in each of these countries.
Canada introducing a beneficial ownership registry is obviously not going to solve the global problem, but it will be a huge step forward and give Canada the basis to push others to do the same.