All the articles on the main English Wikipedia are in "Sentence case". Sorry I don't have a link, but it's what the wiki-style is. If you go into any article on the main English Wikipedia, section names are all in "Sentence case".
Wikipedia is its own wiki, not related to this one. The titles were chosen this way (which I think makes sense) way before I was here. We don't have to follow the Wikipedia or any other wikis style, and this isn't the only difference.
What matters is the having the entire wiki to have a consistent look, since most sections are in title case trying to change this right now would create unnecessary work with questionable benefit.