#2. Total Immunity
Bowser gets to do whatever he wants, whenever he wants, in the full knowledge that everyone will put up with it. He ruins Mario's life almost yearly, and Mario doesn't seem to care. Repeated home invasions and sexual assaults normally lead to restraining orders, not golf invitations.
When you've abducted our girlfriend 10 times, then no, you may not come Go-Karting with us. They even invite him to the things that wouldn't make sense even if he wasn't a repeat sex offender -- call us crazy, but when someone BREATHES FIRE, you don't invite him to the Winter Olympics. It's not like Mario even gets to hit him -- Bowser is constantly being bathed in fire and dropped down bottomless chasms. Drops don't kill things in Mario-land, gravity being somewhat less powerful than homeopathy in that universe, and as for the lava, we've already mentioned how he breathes fire. That's less a fiery end than a gentle magma spa (and the one time it did anything, in New SMB, it turned him into a Dry Bones Bowser, and those things are even more unkillable).
#1. A Fate Worse Than Death
If a sports analyst wants to use math to say Team A is much better than Team B, he'll often say, "If they played 10 times on a neutral field, Team A would win nine times out of 10." If Team A is the Miami Heat and everyone on Team B is in a wheelchair, he might say they'd win 999 out of 1,000. Well, we don't have to use hypothetical scenarios for the matchup between Bowser and Mario. They've played on a neutral field billions of times and, whether you choose to believe that there's a Prestige-style Mario cloning machine just off screen at the beginning of each game or use quantum physics' notion of multiple universes, the fact is that Mario has set out on a quest to defeat Bowser billions of times, and his winning percentage has to be somewhere well below .001 percent.
Mario has died millions, maybe billions of pointless, futile deaths. His incredibly mortal coil is repeatedly flung into everything from medieval spiked pits to relativistic black holes -- everything human technology has ever achieved has been used to kill Mario.
A very mortal coil.
Now granted, most video game characters are slightly more expendable than shotgun ammunition in Texas, but Mario has it worse, because he's the family-friendly one. He's the My First Character for everyone introduced to video games to try controlling. And since he was the character that launched the NES and popularized home gaming in the first place, most of the people who have ever controlled him had no idea what they were doing at first. He's died more often at the hands of children than ants, and was steered clumsily off more cliffs in the 80s alone than the entire history of cars in Ireland.
Add the fact that most casual Marionizers never beat the game, and you've got an endless expanse of parallel worlds where Bowser is not just winning, but winning with ease. In any coherent universe in which Bowser exists, the odds are extremely likely that Mario is either a minor blip on his security radar that went away within 15 seconds, or an army of clones he entertains himself by killing over and over again.
In the end, Mario vs Bowser might be the most unhealthy rivalry the world of fiction has seen since Captain Ahab decided he could take Moby Dick in a fist fight. And even Ahab had a better attack ratio: At least he got to see his nemesis twice, and managed to only die once.
Check out more from Luke with 9 Video Game Easter Eggs That Took Years to Find and Umbrella: The Most Wasteful Movie Corporation Ever.