Maybe 'cause of HTML? Depends on what framework you look at. Web tech is everywhere, so you see HTML alot.
In some senses, I'd say it's more structured and readable that formats like JSON. e.g. compare:<p><div id="parent"><div id="child">Hello World</div></div><p>div: { id:"parent", members: { div: { id:"child", content:"Hello World" } } }