Diatomacious Earth (DE.) filters

The filtration properties of diatomacious earth have been known since the antiquity. D.E. pool filters are largely outdated due to advanced designs of low maintenance high-rate sand filters, special filter media, and efficient cartridge fiklers.

Cost

Pool builders and pool stores will recommend them to new customers because they are expensive and make the best profit - and get a captive customer for D.E. powder. Aggressive marketing for years has monopolised the North American market for some brands for so long that even many pool builders often don't realise or bother to understand how much more time saving, water saving, and environmentally friendly sand and cartridge filters are. Compared to all other filtration methods D.E. filters may well be among the most efficient, but are  the most  expensive by a wide margin.

Maintenance

D.E. filters are the messiest and most highly maintenance intensive of any kind of pool filter. They should be disassembled and thoroughly cleaned once a year and the grids inspected for wear and tear and The cost of replacement grids is high. Replacing the grid cloth may be more economical but is very time consuming. 

Cost/Benefit D.E. vs sand, glass, and Zelbrite

The level of filtration provided by sand, about 15 microns, is more than adequate for any domestic pool up to about 100m3 (26,500 gal) that has a light bather load of an average family and a occasional poolside barbecue party. For heavier bather loads, a sand filter filled with Zelbrtte (a "clinoptilolite" zeolite), will not need the media changing for a decade and with its 3 micron filtration surpasses the performance of D.E. Glass media is cheaper than Zelbrite and will also last for many years. Changing normal silica sand every three years is a day's work but far less of a challenge than taking a D.E. filter apart and putting it all together again. Sand filters cost only a fraction of the price of the D.E. filters which have now largely gone out of fashion outside the US.

Use in commercial and community pools

There may be an argument for using D.E. filters with very big or commercial pools with a heavy bather load where the maintenance budget is not a handicap and  full time staff keeping the pool and its systems running. D.E. powder is a health hazard for the eyes and lungs, ans a carcinogen; people working with it regularly should wear protective clothing and a face mask.