Why do some decks run only one of a card?
The lands that that deck only has one of are generally nice to have if you can get it, but not clearly better than the alternatives.

The deck is entirely white, so Cavern of Souls' second ability is only useful to generate white mana, and that mana can only be used to cast creature spells of a predetermined type (probably Human, given the makeup of the deck). It can be nice to be able to cast uncounterable creatures, but the drawbacks mean that you don't want to have too many, and you probably don't need to cast two uncounterable creatures on the same turn in most cases.

Similarly, the deck color makes Horizon Canopy's mana generating ability significantly worse. It can be very nice to have a difficult-to-counter instant speed draw for one mana, but you can only use it once per land, so it's not worth having too many of them.

In this deck Ancient Den is basically a Plains. However, if you side in Enlightened Tutor, you can tutor for it. It barely hurts the deck, and it's not worth using a slot in the sideboard for it, but non-basic lands are more susceptible to land destruction, so you don't want too many.

The other cards that the deck only has one of are the equipments, but that makes sense because Stoneforge Mystic can tutor for them and put them into play from your hand, so all you need is one somewhere in your deck.

It's also worth noting that this is a control deck; it's designed to slow down the game. Longer games make it more likely that you'll get any particular card eventually, and it's not as important to get some cards quickly.