To say that the Apple TV hardware is getting long in the tooth would be an understatement. The current model launched in 2012, with some slight internal changes in 2013, and still runs on Apple's A5 processor.
Let's step back for a second before answering that question. The reality is that the new Apple TV isn't late: The product hasn't been announced and doesn't have an official launch date. I think, however, it's reasonable to think that Apple did plan to revise the device by now.
Having said that, I doubt there are any hardware challenges holding things up. It's not likely an updated Apple TV is facing component constraints, for example. The product doesn't use cutting edge hardware that's in short supply.
Instead, I suspect it's HomeKit that has slowed things up.
We already know for a fact that the Apple TV will be a gateway for HomeKit devices. That will provide remote access to lights, switches and HVAC controls supported by HomeKit. How will this work though?
Either the remote access will happen through Apple's cloud services or will have some type of network connection beyond the home. My money's on the former for security reasons; I've punched holes in my network firewall in the past for remote smarthome products and it's not simple nor the most secure method.
That suggests that Apple may not have nailed down this feature yet. And it's a key one. People will want to have their A/C or heat come on automatically when they're a few miles from arriving home. The same goes for lights.
Then there's the other aspect of HomeKit: It seems like a bit of a moving target.
Apple's hardware partners have spoken out about changing requirements for HomeKit certification, for example, even as recently as a few months ago. That has caused the rollout of HomeKit devices to be lackluster at best more than a year after the platform was announced. As it stands, there's only a handful of HomeKit products you can currently buy.
Compare that to the $49 Wink hub that I'm now using with an Amazon Echo and you can see the disparity; there are more than two dozen Wink-compatible products ranging from switches to automatic blinds and webcams to thermostats.
Simply put: HomeKit hasn't really arrived yet. And controlling HomeKit products through an Apple TV is surely going to be a big part of the device, so why launch it with a whimper instead of a bang?
There is another key feature of the Apple TV that will build buzz: Cable channels for cord-cutters. Is it possible that Apple is still working out content deals to support that? Probably. But those can easily be added via software: See HBO Go as evidence of that.
Apps are expected for the new Apple TV as well, but again, I don't see that as being a hold up. We're talking about an SDK that won't have many device variables. The software will support 1920 x 1280 displays, regardless of the television make or model, for example.
Perhaps any perceived Apple TV delay is a combination of all these factors. Regardless, the slow pace of HomeKit progress is the main reason in my mind. Content deals and apps are software-based challenges that can be addressed at any time. Rolling out a HomeKit gateway before there are many HomeKit products available doesn't just seem like an Apple thing to do.