The short answer is that it's because of the New World Order, the massive redesign of how complexity was handled in MTG releases from late 2011.
The things that now cause a card to be put at a higher rarity are:
- Strength of the card overall. More powerful cards are rarer so it's harder for someone to just load their deck up with them and crush people unlucky enough to not have those cards.
- How complex the card is to understand, in terms of its actual behaviour. So a card that requires a lot of text, or careful understanding of priority, is more likely to be rare.
- How complex the card is to understand strategically. So a card that has simple mechanics but looks "useless" might be rarer to make you think about what's so special about it (because at a common you might just think it was a rubbish card used to bulk up the set).
- How complex the card is, in terms of interactions with other cards. If there's a three-card combo that results in a crazy chain reaction where you have to carefully pay attention to the order of execution to understand what's going on, at least one of those cards is probably going to be put at rare.
So rare cards are ones that either feel special to get in a booster (because they're that good), or that you have to think about how to best use (and so you don't get cognitive overload with lots of complicated cards at common). In the case of a dual land, it's usually a bit of both. The ability to choose which colour of mana you're producing is incredibly powerful, because it makes it easier to mix two colours in your deck without worrying about mana screw, or to splash a colour you wouldn't otherwise include. Imagine if your opponent was playing an apparently straight black creature deck and suddenly dropped a couple of these. Why would he need blue mana? Is he going to summon a massive creature, or maybe he's got some spell counters available?
As pointed out in other answers, the fact that it can be played untapped means that you don't sacrifice tempo to get the dual land out, either. With many dual lands, the bonus of having that flexibility is countered by needing to wait an extra turn to make use of it (barring some ability to untap a land). So as long as you've already got a couple of lands out, this is a penalty-free dual land, meaning that it's at full power by turn 3 in many cases (and potentially earlier if you can get around the one-land-per-turn rule somehow).