Given that sometimes mobile internet speeds can be slow, depending on coverage, searching using the default search engine might be faster than loading up a different search engine, and then using it to search, which is a multiple step process. Similar to how one might use the search bar built into Android to fire up a quick search rather than launch the Chrome app and then searching through it. However the fact that the number has decreased from 58% to 52% would suggest that Windows Phone users are starting to switch, although it was not mentioned whether they might be switching to either Yahoo or Google.
In fact it was recently that Yahoo tried to make their presence felt on Android by acquiring Aviate, an Android launcher which will presumably come with Yahoo’s search and services installed, essentially overriding Google’s own search and weather services.