Asia is a pretty big geographic area with lots of ethnic groups, that some differ from others should come as no surprise to you if you understand the demographics at all.
Similarly, you would not be surprised that not all "North Americans" are of the same color.
As to the geographic distributions:
wikipedia wrote:Dark skin helps protect against skin cancer that develops as a result of ultraviolet light radiation, causing mutations in the skin. Furthermore, dark skin prevents an essential B vitamin, folate, from being destroyed. Therefore, in the absence of modern medicine and diet, a person with dark skin in the tropics would live longer, be more healthy and more likely to reproduce than a person with light skin. White Australians have some of the highest rates of skin cancer as evidence of this expectation.[12] Conversely, as dark skin prevents sunlight from penetrating the skin it hinders the production of vitamin D3. Hence when humans migrated to less sun-intensive regions in the north, low vitamin D3 levels became a problem and lighter skin colors started appearing.
http://en.wikipedia.org/wiki/Black_people#Dark_skin
As to alphabets, Asia is again much more diverse than you seem to understand and you seem to be limiting your discussion of "Asian" alphabets to East Asian alphabets from countries like China and Japan. Asia on the whole uses many different writing systems.
The "alphabets" (writing systems rather, as alphabets are a particular type of writing system with distinct letters for both consonants and vowels) you refer to are not pictographic, they aren't even true logographies but rather a mixture of a logographic and syllabary writing system.
For example, in Japan Kanji is a logography and Hiragana and Katakana are syllabaries.
As to why they are used it's the same reason a ball is called a ball. A person, or persons, began to do so and there was never sufficient momentum toward anything else.
In other words there's no real "reason" and languages aren't planned. They happen. So someone started writing this way and it caught on, and so far the alternates haven't caught on enough to change this. And that's the answer to the "why" for almost any question about language ("why" used to drive me nuts when teaching languages, there's no answer to why at some level because all words are arbitrary descriptors at some point).