That's the point. Where is the Delve format now? Is it in use? Are we seeing it in any of the 5e adventures? Oh, right. No we aren't. Why not? Because it was immediately dumped on by the fandom that hated it. So, out the window it went.
Just like any other time WotC tries to do anything different. I mean,
@mamba didn't even know what it was. That's how much of an impact it managed to have.
Tome of Magic was mentioned before. Hey, I adored that book. I still pine for the days when I could play a Binder. There is an excellent 5e version from Head of Vecna that I keep pitching to DM's to let me try out. But... again, what happened to those three classes - the Binder, the Shadowcaster and the Truenamer? Oh, right, the binder got stripped down to the bare bones and became the 5e warlock without pretty much any of the actual innovations that we saw with the Binder. The Shadowcaster and the True Namer have disappeared like the dawn in the morning.
So much for innovation.