The daytime 'lifestyle' market is already well served by ITV and it's so typically mainstream. C4's remit is to tell untold stories, take creative risks, drive innovation and to take more risks than other channels, not just to replicate them. Having a chat about the new series of Bake Off doesn't exactly fit with this remit.
Well, C4 did do this in the nineties with Light Lunch, in the same slot too, which felt like a proper alternative to the other channels and enjoyed a real cult following (though it was never the same after it moved to teatime). You could argue that the challenge for C4 is that what used to be alternative has now become very mainstream, and the other channels have been influenced by them, so it no longer feels particularly different.
It's like The Big Breakfast, when that started it was absolutely light years ahead of the alternative, but it was so influential that you could see its impact on other shows. Don't Forget Your Toothbrush is another example, when that began it was a big show and seemed much fresher than the tired old Saturday night output on BBC1 and ITV. But now people like Keith Lemon and Alan Carr are on primetime ITV.
I like Steph a lot, and I think it's underselling her to say anyone could be a success on Breakfast, she had a really likeable and distinctive personality. But she does need a format that's more than just Steph doing stuff, in the same way Slap Bang was a flop because it was just Ant and Dec doing stuff.