If the alleged victim isn't in the country, refuses to travel here for trial (no victim no trial) and expresses a desire to move on with her life, the crown won't pursue charges. Especially if other things have happened behind the scenes that we may not be privy to (e.g., anger management classes). None of us know what happened that night, and I didn't even read the "reports" on the incident because there's no way to verify them (and any leaks would be highly illegal). So I'm not making a judgement either way. But if we're forced to make one based on some unrealistic scenario, it's likely simply based on statistics that an assault occurred.
Although, also based on statistics and this is something that the media doesn't report on, I wouldn't be surprised if it was a mutually abusive situation. Research actually shows that men are more likely to be abused domestically and more likely to be seriously injured. Of course, that doesn't excuse anything that might have occurred on Osuna's end. And I'm not expressing myself well here, I don't mean to point fingers at Osuna's girlfriend, I'm merely making a ham-fisted attempt at explaining the statistics on domestic abuse that exist from a perspective that isn't often reported and may or may not apply to this incident.