HTML is a standard that can be easily represented in an application that implements the standard.

XML is also a standard, but I a random application that supports 'XML' doesn't necessarily know what to do with the information in the XML file. I have a lot of problems with XML, if someone gives me a 'foo.xml' file there is no easy way to figure out whether it is the configuration file of some application, or a data file for another one, or an interface description for the third.

What makes you think that some random SOAP client would actually be any useful? My guess is that it would require implementing something that actually understands the data encapsulated in the file, in which case you could just as well use an XML or HTML parser.