The same reason Geoff Johns brought back all the pre-Crisis stuff he grew up with when he was running the show at DC. It's what the folks in charge grew up liking and, to them, it's what's classic and great.
The 7th Edition DMG has a good chance of including Eberron in it, once Millennials and Gen Z folks are fully in charge over at WotC.
I do think WotC's weird indifference to creating a setting laser-focused on their young audience is a mistake, although third party productions like Obojima are definitely benefitting from it. Hopefully one of their new settings coming in the next few years is more in keeping with Avatar and Studio Ghibli, etc.