Bowser’s stated goal, according to Wikipedia, is to “marry Princess Peach, defeat Mario, and conquer the Mushroom Kingdom”. Not having played the Mario games very much (read: at all) and being unfamiliar with the Mushroom Kingdom’s economy, it seems to me that Bowser’s pursuit is financially quite unprofitable. By kidnapping Princess Peach, Bowser then has to deal with the inevitable journey of Mario and Luigi and whoever else through his lands, which seems like it would be a huge financial drain on him.
|What, you think upkeep for a lava castle is cheap?|
-It’s not made clear, and probably differs depending on the game, whether the resources Mario and Luigi find throughout their various quests—mushrooms, coins, fire flowers, POW blocks, etc.--are part of Bowser’s arsenal or simply indigenous to the Mushroom Kingdom. If it’s the former, he’s losing a colossal amount of money as Mario goes through his power-ups; if the latter, he’s still losing money because they’re gathering and consuming these valuable goods within his territory that he can no longer harvest.
-Bowser has to employ whole armies of Koopas and other enemies to try and stop or slow down Mario and Luigi. That means regular pay, hazard pay, compensation for injuries suffered on the job, medical care for the disabled and death benefits to the families of fallen soldiers. Bowser could be simply intimidating the Koopas into helping him, or else relying on clan loyalty, but Koopas have to eat like everything else. And after the first few games where Bowser is defeated, you’d have to think his intimidation factor among the rank and file would go down. It’s a whole lot easier to ask for medical benefits when your employer isn’t an invincible tyrant, but a vincible one, and some asshole has been bouncing on your head and throwing your shell at people all day. And somebody has to water those damn carnivorous flowers.
-Property damage and loss. Bowser doesn’t own all the territory Mario travels through, but he tends to own quite a bit of it. That means a considerable amount of time and money spent constructing spiked passageways, digging lava pits and installing huge swinging blades to screw with Mario and friends. And every time Mario clears out one of Bowser’s castles, he raises a different flag over it (at least in the game I just played part of). Bowser is basically a warlord who is usurping parts of the Mushroom Kingdom, so it wouldn’t be surprising if the raising of a different flag meant that it was reverting to government control, or at least remaining in Mario’s hands. Either way, Bowser loses that castle—and because his armies are eventually decimated by Mario and he himself defeated, he probably doesn’t have the strength to retake them by the end of the game. That’s a big deal.
-Finally, the loss of prestige is important. Sure, Bowser is big and scary in all the games, but after the first few he’s a known quantity. Mario and Luigi know him, they’ve worked with him and they’ve beaten him. He’s not a tremendous threat. This relates to the “intimidation factor” thing up above: by being beaten, he loses prestige and reputation amongst the rest of the Kingdom. And that means that he is less able to demand concessions or tribute from the central government. Why would they give anything to him if they know he can be trounced by a couple of unemployed plumbers, even if he has kidnapped Princess Peach? After the first few times, she’s probably gotten used to it by now. And there’s always the pain and suffering that Bowser endures in the process of repeatedly getting his ass kicked, including getting all of his flesh burned off and turning into a skeleton.
|"AAARGH! Oh, but it's OK, I'm going to marry Princess Peach and--oh, God, how are my bones staying together."|
It's hard to think of what rewards the Mushroom Kingdom would offer that could be tempting enough to go through all this crap, game after game after game. At some point in the last few decades, Bowser's total expenditures probably overtopped the predicted economic productivity of the entire Mushroom Kingdom if he was able to capture and hold it. When it gets to that point, buddy, you should probably just give up.