I have asked similar questions on the Novosibirsk before.  However, few people are interested in it on the forum...as seems to be the case with half the openings I play 

I can only assume that everbody is more at home in the better explored and fully equal positions of the Chelyabinsk....I don't actually think the early Nc2 before castling is too bad for black....counterplay is just less violent than in other lines.

When Shirov played Carlsen (he explains that in detail in a video on CBM 131) he was or he rather put himself into an absolutely must-win situation: it was the last round game. And he was ½ point behind Carlsen and wanted to try everything to win the tourney and to get the seat for the Grand Slam Final. And according to his explanations he felt he should go for maximum complications (and not necessarily the objectively best continuation). I think, this explains his choice. And Shirov was lucky that it worked fine for him.

Topalov, however, was a bit too late and his opening choice (i'm sorry to say this because he has many great ideas) was just a bad copy of Shirov's. And Magnus came perfectly prepared this time (at Nanjing)...