Nothing inherently makes the Discourse frontpage more relevant that something else. Google can do what it does because it considers all the links on the whole internet. Discourse isn’t building a map of the internet, but doing text search. It can’t know the difference between all the uses of the word “bear”. It can’t ignore idioms. And when you limit searches to only titles, then it has a lot less to go on - it can determine “relevance” based on the number of times the search terms come up in a whole post, but when you limit it to titles then lots of titles will only have the search term once, and so will be ranked equally.
That said, the “Most Viewed” sorting might help.