Because the Star Forge robes were game mechanics, nothing more - plus, when he was captured and put in stasis, it was at least 4 years post-KOTOR. Even if he did have them, hypothetically, they tend to stand out. Most of the novel he was trying to fit in - first on Coruscant, then among the Mandalorians, and finally on Dromund Kaas.
But yeah, game mechanics of the original game. Do you think Revan would really pause on a mission that important as killing Malak, only to make himself some clothes? Nah. He wore what was on hand - and the reason he was wearing his "classic" outfit in TOR is probably because, non-canon wise, it is his most widely-recognized outfit, and canon wise, Scourge brought Revan and Meetra changes of clothes - and Revan's robes seem to be Sith style, even three hundred years later. Look at all of the Inquisitor robes you can get in-game that are recolors of his robes - whether red or blue or black. Where do you think Revan got those robes in the first place, as a Sith? He was corrupted by the Emperor on Dromund Kaas, and he was probably given new robes to wear in honor of his 'Darth' title - though he kept his mask from the Mandalorian Wars.
And besides, I am not a giant fan of the Star Forge robes, so I wouldn't have wanted to see him wearing those anyway.
I don't think the Starforge robes can just be written off as game mechanic. The Star Forge makes thes robes for you, they're entirely unique, and they are the most powerful lightside robes in the game. They aren't an in game secret or anything, they're practically part of the path through the Forge.
I loved the Star Forge robes and, since the whole reason they included Revan in the first place was for us to fanboy over him, I think it would of been pretty cool to see him wear them.
I haven't read the book, I've heard it's pretty bad so I keep procrastinating on it.