Because most fans of SNSD are in Asia. Yoona fits the Asian beauty standard perfectly. She has the ideal face of every Asian - big, cute eyes, very pale skin, v-shaped face, good nose, good height, and it's also all natural beauty. Yoona's "v-shaped face" is the ideal face type for all Asians, and Yoona has it naturally, so they call her a goddess.
Hyoyeon doesn't match the Asian beauty standard. However, internationally, the most beautiful ones are considered to be Tiffany, Sooyoung, and Hyoyeon (and Jessica).
Personally, I think:
1. Yoona
2. Tiffany
3. Sooyoung
4. Hyoyeon
5. Seohyun
6. Taeyeon
7. Yuri
8. Sunny
I don't think Yuri is pretty at all. Her face is boring, bland, etc. but she has the best body! In Asia, Yuri is one of the prettiest because she resembles Yoona a little, so she fits their standard.
I know internationally it's like:
1. Tiffany
2. Sooyoung
3. Hyoyeon
4. Seohyun
5. Taeyeon
6. Yoona
7. Sunny
8. Yuri
The last 2 can switch around.
I am an international fan, but I am Yoona biased so I think she's the prettiest<3