Tim Ewald, as usual, is absolutely right:
Today, SOAP over HTTP is the only transport that works with two different toolkits at the ends of the wire. Until we have that with other open protocols, HTTP will dominate.
Well, almost absolutely (since HTTP is not a transport, as Mark Baker would be quick to point out). In any case, using HTTP has so many advantages even if it is mis-used as a mere transport that it’s hard to see why using something else for heterogeneous integration might be worth the hassle.