Why Are Mario and Luigi the Only Guys to Save Peach?

So, Mario games are slightly popular, aren't they? And they've all got a pretty similar plot. The plumbers (/tennis players/worst doctor ever/star football players/ party organizers/ morally questionable zoo owner/ Mario has far too much time on his hands, and is never actually doing what his actual job is) are hanging out with the princess, and then she gets kidnapped. Instead of the castle guard sorting it out (which... seemingly doesn't exist? Seriously, you'd think after getting kidnapped... ten times? Peach, it's like you want to get kidnapped.)

And you can hardly say you're defenseless. You
fight pretty well in Galaxy 2 and your own DS
games. Heck, you can fly! And you have superpowers!

Peach's security issues aside, why is it that when she inevitably gets kidnapped, it's the role of some plumbers to go save her?

Because they're the only ones who can. They're the heroes Mushroom Kingdom deserves, not the ones they need.

It'sa me! Batmario!

The main groups that live in the Mushroom Kingdom, and aren't on Bowser's side are Toads/Toadettes and Yoshis.

There is no way that the Toads could save Princess Peach (yes I know, they did in Galaxy 2, I still don't know why), because, simply put, they'd be instantly stopped. In the first game, Bowser has something that's not really explained that can turn any Toad into a square made of bricks (that's an... oddly specific power, I must admit). With those sorts of odds, they probably wouldn't have volunteers lining up to go save the princess. And if there are people who could do it without a near certain chance of doom, why not use them?

That leaves Yoshis (Yoshies? Yoshii?). But they only really started to exist in Super Mario Bros, by which time the brothers had gotten pretty settled in their part time job of princess saving. So it could be that they didn't actually live in the Mushroom Kingdom until after the events of that game, where they would realise that the mild protection that comes from being in a kingdom with Mario and Luigi is worth leaving their own society.

So, Mario and Luigi are basically the last bastion of defense for the Mushroom Kingdom and most of the (somewhat) good aligned world?

Oh gosh, imagine if Bowser attacked the Mushroom Kingdom whilst Mario was busy being wrongly accused in Isle Delfino. Good job guys, you threatened the safety of a kingdom and all of the Toads because you couldn't be bothered to notice that the guy that arrived literally that day probably didn't commit the crimes that had happened earlier. It's not like he could time travel or anything...

Okay, I guess he can.