Currently Online 
I think it has to do with the audience, to some degree.
However another reason might just be what G2ThaUNit said.
Its a side console in alot of cases, that people get, to play nintendo games.
If nintendo started putting their games on playstation and xbox, it would have a drastic effect on how many units (hardware) they sell.
simply because I think, theres more people that own a nintendo as a side peice, than with the other two.
"What would it take for 3rd party sales to be larger on Nintendo Systems?"
To deal with the issue that G2ThaUNit mentioned, it would require that Nintendo went back to a traditional console.
This would mean the end of the switch line. So it would come at a huge cost, and fly straight in the face of whats proven to work really well for them (blue ocean strat, ei. be something the others are not)
Also I think Nintendo is the only one of the 3, that actively views multiplats like competitors for game sales.
While Sony and Xbox, are like "nice free 30% cut", lets get all the multiplats.
Nintendo makes the system for themselves, their hardware is for their own games, f*** other developers and their cares.
Why would we want the competition for game sales? we make more selling our own games. (the Wii U changed that abit, but its there, and the more successfull Nintendo is at its current strat, the less likely it is to change)
Thats why you hear stories about nintendo being hard to work with ect from devs.
While Sony actively goes out to talk to dev studios, and what they want and need in future consoles before even starting console hardware design ect. They sent out dev kits and have people come and help, learn to use it and the programming and such.
So its also partly a mentality shift.
Xbox has the same issue as well to a degree.
Theres games that have been playstation exclusives because xbox guys couldn't bother responding to emails and issues devs have had.
"we re stuck at this problem, please write back" *crickets*
