MMP is difficult to explain to anyone uninterested in electoral reform, ie the majority of voters. Include things like party lists and members at large, and you can get some pretty significant drawbacks. There was also the more likely possibility of constitutional issues than with STV or ranked ballot, given the seat allocations outlined in the constitution.
Ranked or STV are easy to explain, ranked especially. Ridings and the ballots don't even need to change. Instead of an X, put numbers in the circle. Easy-peasy to explain.