This Death and Taxes deck, for example, has 1x Cavern of Souls, 1x Horizon Canopy, and 1x Ancient Den. Since it's a 60 card deck the odds of drawing any one of them is 1/60. ie. quite slim. So why do they include cards they likely won't be drawing? Why not run 4x of each card? Gives you less versatility, I suppose, but what good is versatility if you don't draw the cards you need?
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.
First, a recommended article about singletons in general:
In this specific case, the cards you mention are all cards such that the deck never desperately needs to draw them; the player might think "I want a land that taps for white mana" but probably not "I want Ancient Den." They may, however, think "I sure wish I could get a land with this Enlightened Tutor."
Furthermore, while playing multiples of a card increases the chance you draw one, it also increases the chance that you draw more than one, which can be bad. All three of these lands have drawbacks such that you may not want to draw a bunch of them. The Horizon Canopy makes you lose life when you tap it for mana, and cycling it takes time. Having the one might be a hedge against mana screw or mana flood but drawing them all at once can either eat a game-losing amount of your life total or force you to waste time cycling them. Similarly, you may not want all your lands to be artifacts depending on what you're up against, and Cavern of Souls can only be set to a single creature type.
The rest of the singletons are all tutorable (with Enlightened Tutor and possibly Stoneforge Mystic), so the builder will be able to tutor for or topdeck them. The only exception is the Sunlance which probably isn't so much "Sunlance #1" as "kill spell #5" (in cases where Sunlance is useful, Swords to Plowshares is probably also useful).