It's been said already, but mappers choose on their own accord what songs they want to make a map of and get it ranked. Mapping and the ranking process is a long, drawn out process that involves hours and weeks of effort, so it's extremely inconsiderate to expect someone to go through all this for any song other than one they decided on their own and are wholeheartedly engaged with. That means any song you could suggest to someone to map isn't gonna meet their standards.
If a song is really good or popular, chances are a decent mapper will have picked it up. If not, that just means it's not happening. Mappers don't always share the same song preferences as players, so there are plenty of discrepancies (see the absurdly high amount of Japanese music on osu).