One of the reasons is because of low head. Using a high head pump on a low head loss pool will result in the pump not having enough load on the impeller. Not only is this inefficient in terms of energy usage but it can also burn up the pump. For most pool pumps, they will draw the most power with the lowest resistence which is like operating on the right most side of the head curve.
When picking out a pump for any pool, you want to make sure you are operating in the middle of the head curve where efficiency is the greatest. For high head pools (IG), this is between 45-65 feet of head. For low head pools (AG), this is usually below 40 feet of head.
Here is a good reference for the difference:
http://www.poolplaza.com/inground-aboveground.shtml
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