Obviously, the Wii being light-gun friendly is a big help here.
But that's not the only reason why these games have come to Wi only, and proven to be better than many would have expected. Umbrella Chronicles really was surprisingly good (within certain common sense limits, of course), and so, apparently, was the recent House of the Dead: Overkill and I understand a Deadspace game is doing the same thing on Wii now.
It's because on-rails games are system-efficient. The Wii is not very powerful and cannot handle fully open games. It can handle them on-rails.
Obvuously the other systems can too- but the point is this. Developers do not WANT to limit themselves to rail games if they don't have to. On other systems they don't have to. Rail games are kinda yesterday's thing.
And so you end up with a situation where, by being less powerful, the Wii actually becomes the best platform to extend the concept of what you can do with an on-rails shooter- in exactly the same way that the DS has seen good quality sprite-based platform games like the continuing Castlevania series or, (with some quasi 3d), New Super Mario Bros. You can't make it free-form, you can't make the graphics hi-res and all that... but what you can do is try and find ways of simply making it a better game.
Any other system COULD do sprite-based platformers... but no-one wants to leave so much of the tech laying unused and fallow like that. The games are good games that any system could do, but only DS has them.
So it is here. Any system COULD have on-rails shooters, but no-one really wants to do them on systems that can do more, technologically speaking. On the Wii, that's the limit, and people have made what they can of it.
So as I say, the DS had already showed it- sometimes, a system's technological limitations actually encourage people to do decent stuff that otherwise would not have been done.
Of course, those same limitations also means that lots of stuff is NOT on them either, so owners of other consoles can't really complain.