Great question....
I've always thought that canisters are expensive "pain in the ass to work on" filters that are not much better than hang on's or power filters.
I would have to guess that they are more expensive to manufacture and are of a better quality & design but by and large, canisters do the same thing a power filter does. They may be able to hold more media and perhaps move more GPH than a similar power filter.
Also, the manufacture may not make as much money on "replacment cartridges" with canisters when compaired to power filters so to counter that they may charge more money for the filter. (that's a guess though)
Not a big fan of either style filter........