Because it’s the most popular. Also most voters wrote their reasons in the topic already.
I’m not sure what to define as saddest. The work that made you feel saddest while reading it, the work that made you cry the most, the work with the saddest ending thus disqualifying any works with happy endings, the work you feel saddest remembering? It makes it difficult for me to decide.
It’s a toss up between Clannad and Air, both of them are good contenders. In the end though, I think I’d go with Air.
I changed my answer to Little Busters. When I read Refrain yesterday, it absolutely destroyed me. I cried at least 5 maybe 6 times. I have never cried that much before. Clannad After Story only had me cry once even though there were many moments that made me tear up. I cried when Masato’s episode reached it’s resolution. I cried when Kengo wished that he had spent more time with the Little Busters. I cried when Masato calmly said his farewell. I cried when Kengo accepted the end of that world. And finally I absolutely lost control when Kyousuke turned around with the tears in his eyes. Never has it felt so real to me. Never have I appreciated a story so much. I will forever treasure Little Busters in my heart. Even though Kyousuke said no more crying, here I was, sobbing uncontrollably in front of my very, very confused roommate.
Well currently i’d say CLANNAD with approx. 3 cries when i watched the anime and some… 6? more when i read the official english release of the VN, which rounding up gives a grand approximate total of 10 cries. However i am yet to read and reread a few so my opinion is bound to change.