I think the speed of the experience is core. Browsers on mobile eat up a big part of device's resources, and the time it takes to load it and perform the search is so big that it doesn't make sense to use it for anything else than general web search. If "there's an app for that", one will use an app for that.
Just as a counter data point, if there's an app for that I rarely use it. Because usually I prefer the mobile browser version to the app for many reasons, not least of which is the way to get to the web version is always the same.
