The constitution requires that a province’s representation in the House of Commons cannot be diminished, which means that the number of seats allocated for that province cannot be reduced.
For example, Prince Edward Island is guaranteed 4 seats in the House of Commons. Simple MMP preserves these 4 seats by providing 2 Local MPs and 2 Top-Up MPs from PEI.
One can produce a technically superior proportional representation by having a national pool of top-up candidates rather than 10 provincial pools (territories do not require a top-up, as they have only one seat each). However, this kind of system would require a constitutional amendment, since provincial representation is not guaranteed.
Simple MMP uses provincial top-up pools, and therefore satisfies the constitutional requirements in spirit and in letter.