Because putty is smaller, faster, can work with serial ports too, which lets you use it to configure routers, debug arduinos, etc. It looks nicer, is easier to

Because putty is smaller, faster, can work with serial ports too, which lets you use it to configure routers, debug arduinos, etc. It looks nicer, is easier to choose fonts, offers logging, offers easier clipboard control, and a lot of other features. In short, it's just a much more useful program.