This is because:
They are easy - you just blend fruit, leafy greens, and water.
They are fast - They take less than five minutes to make
They are alkaline - Alkaline is what many people are after, especially if there are health issues. Leafy greens are some of the most alkaline foods in the world and drinking a lot of them gives more of that quality to you. Alkalinity is associated with flexibility and calmness. Alkaline foods are usually those with more alkaline minerals like magnesium and calcium.
One very prominent aspect of drinking green smoothies is less cravings for other foods. Drinking highly alkaline, easily absorbed foods and juices often reduce cravings significantly because they are so energizing and give so much nutrition.
If you don't know where to start in your health journey, green smoothies are a great starting point.
To make them - Put as much greens in a blender as you want - Start with less or around a handful in the beginning - and fill up the blender with enough water to cover the greens.
(If you are using a vitamix or other similar blender you can blend all the ingredients at once without the extra step of blending the greens and then blending again after adding fruit)
Blend for around 10 seconds. Now add as much fruit as you want and blend again for another 10 to 30 seconds, until it looks completely blended.
The drink should be green but somewhat sweet tasting. If you use fruit that you like, the drink will likely taste good overall.
Many people drink around a liter a day and experience significant health benefits. For people trying to lose weight, green smoothies are a powerful tool.
For those who want more information on green smoothies check out:
Any book by Victoria Boutenko including Green For Life. She also has many youtube video's online which talk about and show how to make green smoothies.