Two reasons:
- The search bar is optimized for full title searches, so partials will be much less precise.
- "Godot" is a very generic word on a site like Itch: lots and lots of projects are going to have it in the title; yours probably doesn't rank as high as others.
Either way, the search bar doesn't look at tags. For that you need to use tag search, like this. Even better, use the Metadata->Engines and Tools tab in your project settings instead, since that's the recommended way nowadays.