Because you really don't get all the benefits of a native app with a web app. It's getting better, the gap is closing, but it will be a while before web apps are an enjoyable an experience as a native app. I think Sparrow was actually a good proof point of that.
More than that - apps like Sparrow move the goalposts actually. It was such a superior to experience to even the best native clients of the day that it widened the gap between web and native once again.
sometimes I'm surprised by this sort of thing, but then I remember that Google is INSANELY GOOD AT SEARCH and I stop being surprised when that plays out.