Good question, and there could be only one answer, Yahoo! are idiots.
@musicseawater Symbian is the OS powering 40% of world smartphones, dominating the world wide smartphone market since 2000 when it was launched. It powers (or powered at given times) smartphones manufactured by Motorola, Sony Ericsson, Samsung, LG, Panasonic, Sendo and of course, world's market leader, Nokia.
It is not only the most common OS, but also world's best mobile OS, optimized for mobile devices with power management in mind and amazingly efficient user interface, althoug not as cute yet as that of iPhones (but better than that of Android, certainly so when it comes to non-touch devices)
By ignoring such devices Yahoo ignores 40% of the smartphone market, largely outside US. On the other hand, one doesn't really need a dedicated app on a Symbian device, the browser gives you access to all the features you need (unlike on iPhone apparently, since a dedicated app is needed for it).