Because the two situations are different. Electors living abroad can provide proof of residence in the country where they are currently living. But it would likely be difficult for them to provide proof of an address where they lived 10 or 20 years ago.
We were told very clearly that if the elector is able to provide proof of where they lived in Canada and if their parents or sibling, say, still lives there, that information will remain on the register as long as there hasn't been a change. If the person comes back to Canada and moves to another riding, that's one thing, but until then, why not keep the register as it is now in order to make it easier for these electors to vote rather than harder, which is what Bill C-50 does?