Its not because of lack of resources, it's because of lack of customers. They'd not make enough money on the laptop to justify it.
I'd wager it's neither of those. It's a lack of will.
They could sell enough to make it profitable. And if nothing else, it would show some dedication to the "Pro" market in a time when it's becoming questionable. Market share is good for the platform.
It's the same with the Mini and MacPro. They could easily update them with 2017 components rather than selling 2013/2014 hardware for 2017 prices, but they can't be bothered to.