Because Mass Suggestion hits up to 12 targets.
That makes its usability, well, 12 times better than regular Suggestion. Consider a fight against a group of enemies. Mass Suggestion can target possibly all (or at least a large portion) of them. You could turn a villain's thugs against himself. Suggestion could not do it.
Because it requires no concentration.
You can take damage at will without losing the spell's effect. You can cast other concentration spells while Mass Suggestion is working. This also implies that you can have several Mass Suggestion spells active at once, as long as you have a long rest between one act of casting it and the next.
Longer range and duration.
It also has double the range and lasts 24 times longer (24 hours is a really long time).
Level
The level 2 Suggestion automatically fails against a Dispel or Counterspell, but the level 6 Mass Suggestion forces a DC 16 (10 + spell level) save to negate it (props to Joel Harmon for that).
However
Mass Suggestion is indeed more expensive than Suggestion. For Warlocks, you can only cast Mass Suggestion once per long rest, while you can cast several Suggestion per short rest.
D&D Sources
I think Weckar E. is basically correct in that this confusion stems from the fact that, like many elements of D&D, the fey courts are cobbled together from a mishmash of not-terribly-consistent folklore. There seem to be two competing views:
- The two courts are basically mirror images of each other - the Seelie project an image of light and the Unseelie one of darkness, but under the surface they are pretty similar, with members of both courts being fickle and beauty-obsessed.
- A view with more overtones of class warfare, where the Seelie are beautiful and exclusionary while the Unseelie are nightmarish and inclusive.
It sounds like more sources tend toward the first view, but I think there's enough material provided for you to choose whichever one you feel is right for your game.
Other Sources
Other depictions of the two courts that I'm familiar with tend more toward the first view. Often fey from both courts are equally dangerous in their own ways - they merely express their unpredictable personalities and the power of nature differently.
The Dresden Files novels, for example, play up the Summer and Winter connections. The Seelie are bright, colorful, and generally pleasant enough on the surface, but they can also be quick to anger and terrible in their power, like a summer's day giving way to a powerful thunderstorm. Alternatively, they might simply get carried away or be otherwise heedless of the consequences of their actions, turning from pleasant warmth to a dangerous heat wave.
The Unseelie are cold and vicious, but aren't entirely without compassion. After all, winter still has its mild days; it is not an unrelenting blizzard. They still experience some form of love, or at least emotional attachment, and while they might see mortals as playthings most of the time, they may develop a certain admiration for mortals who prove themselves capable of keeping up. Even then, mortals must remain on their guard - it's not uncommon for the Unseelie to have a sadomasochistic streak, and their version of kindness might not seem so pleasant to others.
Best Answer
You're limited to two dice per use, but five dice per long rest
There's rules in the feature about only using half your dice, but not in the context of a long rest:
So at level 5, the druid would be permitted to spend 2.5 dice per use, which we round down to 2 because of the general rule about rounding:
So between Long Rests, a Druid could use this feature five times, spending 1 die each time, or three times with 2 dice for the first two and 1 for the last, or any other permutation they choose that obeys these restrictions.