"Stories too broad and too deep for the small screen." That was the rationale for the New Adventures from the very beginning, confirmed by line editor Peter Darvill-Evans in his introduction when he said the new series of books would "continue the trend of recent seasons of television stories towards complex, challenging plots with serious themes."
But a better description of Timewyrm: Genesys comes at the start of that paragraph instead of the end: "Here is an introductory word about Doctor Who - The New Adventures: Continuity." John Peel, author of the first NA, seemed to take this statement as a license to shoehorn in as many references to old episodes as humanly possible. The Doctor receives a message from his fourth incarnation to start the story, mentally regresses to his third to finish it, and in between spends his time reminiscing about as many old adventures as John Peel can fit in without utterly losing the thin thread of the plot. It's books like this that led to Craig Hinton coining the legendary term "fanwank". (Fanwank. n. Continuity that serves no purpose within the story in which it is placed, and is there merely to satisfy nostalgia or settle a point of fan debate. We will be returning to this many times, so I figured I'd get the definition in now.)
Still, in some senses, Peel did produce a story that could not be told on television. But that's true only in the narrowest sense of the concept; Genesys wouldn't fly on TV in 1989 more because the Girls Gone Wild pixelation technology hadn't come down in price enough for the BBC to afford it than because it was too mature and challenging. The story feels like a mid-to-late Hartnell episode, with the Doctor and Ace (neither of whom feel particularly like the characters we're familiar with) going back to an Important Time in History and learning about Life the Way They Lived It Back Then while fighting a Sinister Alien. The only real difference is that they make smutty jokes about temple prostitutes and Gilgamesh raping all the women he meets. (Get the hot water ready, by the way. You'll need a long shower to feel clean again after the Doctor tells Ace to ignore Gilgamesh's sexual assaults and keep an eye on him.)
When this novel first came out, all the way back in 1991, I skimmed it briefly and put it back on the shelf, dismissing it as another juvenile tie-in novel on the level of the Forgotten Realms books I'd just gotten sick of. I've later changed my mind about the New Adventures, but I think I was right on the money about Timewyrm: Genesys.