Why are fewer tags shown in Potlatch 2 than in JOSM?

Do you mean when you click Edit on the website and select the item it shows as Unknown? If so, then not all editors have the same set of preset tags. Nor do any of them contain every possible tag - it is always possible to invent a new one if the existing ones don't cover something. In the editor (Potlatch 2), you can switch to Advanced mode to see the actual tags that are on "Unknown" items.

Because no-one's contributed a preset for that particular thing to Potlatch 2. (As Ed says, you can, of course, always click 'Advanced' to see the raw tags.)

If you'd like to help by contributing the preset, the potlatch-dev@ mailing list will be happy to tell you how.

It sounds from the comment like you're describing a man_made=water_works. If you're mapping somewhere where water is cleaned between a e.g. river and a place that it is used, then that would be the tag to use. According to taginfo, it's quite widely used already.

Somewhere that handles waste water and treats it before e.g. putting it back in a river would be a man_made=wastewater_plant.

As scai has already said though, that doesn't mean that it'll appear on the "standard" map (although names will render - here's an example). That's simply because there are more features than can possibly be displayed on any one map.

You might also find this page and its corresponding discussion page helpful - you can search taginfo to see how widely used the items on there are.

The main map doesn't show any possible feature, not even all map features. Yet it is still worth to add them because other tools can make use of them as for example search engines (like Nominatim), routing engines and so on. Besides you can also request new features to be rendered or check if the list of OSM based services contains one or more browsable maps showing the feature you are looking for.

Another option is to render your own maps and decide for yourself which features to show, for example by using Maperitive.