Multicolored lands for a rainbow deck
This question is kind of the mirror image of this one: "Why would you want to play with a deck that had 3 or more colors in it in Magic?"

Playing multi-color decks is desirable because it lets you make a powerful and versatile deck by cherrypicking the best cards (for your strategy) from multiple colors.

So, why are 4- and 5- color decks so comparatively rare?


A lot of the strongest spells of a color have deeper mana requirements.

Make a list of your favorite powerful cards. A lot of them probably have costs like 2WW rather than 3W, huh? This drastically alters the math of consistent color access. If you look at this number-crunching article by Frank Karsten, you'll see he figures 12 sources are enough to consistently cast a 2C-cost spell on turn 3, but you need 19 sources to get the same quality of access to a 1CC-cost spell on turn 3. 19! 

This is an active, purposeful design decision to make these cards easier to cast on-curve and on-time in a deck that's dedicated to those colors, and harder to include in a deck that only splashes a color.
Cards are strongest when you play them on time. A mana base that's heavy on comes-into-play-tapped lands or requires casting land-selection spells like Borderland Ranger to set up your mana introduces notable lag before you can start playing spells that will really interact with your opponent.
Opponents can disrupt your mana base. Consider how much tighter mana requirements become when you're playing against a deck with 2x Rishadan Port and 4x Wasteland.


All these serve as a damper on the greediest multi-color decks. On top of that, because:


The color's "pie slices" overlap (e.g. green or white can both play effective enchantment removal, so you only need one of those colors to get that ability in your deck).
Not every color in a format will meaningfully support the specific synergies / strategies you're trying to exploit. (Maybe there just are no red cards worth playing in your midrange deck, even with perfect mana.)
A stable 5-color manabase can lock you out of playing amazing utility lands like Academy Ruins.


&mdash; there's often very little to gain from building a full four- or five- color deck instead of one that's "just" three colors.

Exceptions, such as Chapin's Cloudthresher + Cruel Ultimatum Lorwyn control deck (example decklist), are typically the result of slower formats with multi-lands that support greater color depth than simple dual lands.