Why? Because JT9 is the best weak signal mode there is. On 10 meters I worked with 5 watts JT9 CX2AQ (11.371 km) and LU8EX (11.445 km) I read at PE4BAS that KB2HSH published a nice list about the sensitivity of digi modes. Why should anybody uses RTTY for instance?
JT9 and JT65 are the most sensitive ones. I prefer JT9 because it has a very small bandwidth. Ideal for crowded band activity. At the moment activity is a bit low with JT9, so I call all radio amateurs who like to work QRP or low power and wanted to do some experiments: use JT9. Download the software from this web site of K1JT Joe Taylor.