There's lots of similar examples in other creative fields - artists who have one piece of work that does tremendously well, but everything else they make is only noticed by their more dedicated fans.

My interpretation is that it's because they make the art for them, not for the audience. In a sense they got lucky that one piece of their work found itself at the right confluence of factors to blow up like it did, because that's not something you can really control or guarantee, unless you play it extremely safe (which is why all the big game/movie studios produce sequel after sequel, or old rockstars will just keep performing their classics).

There's an interview with Jon Blow I heard recently that stuck with me, where he says that he could probably have made lots of money making Braid 2 or The Witness 2, but that it just wasn't interesting to him.

Willingly leaving money on the table, now that's the mark of an artist.