Marketing issues? Poor open-source drivers? Lack of Linux hardware enablement at launch? Nothing attractive about Unity? Nope, not any of those, it all comes down to one feature at this time: NVIDIA Optimus. That is the company's (they ask not to be identified at this time) biggest issue right now with how NVIDIA and other stakeholders are pushing hard for this multi-GPU technology in new notebook designs. Under Linux, the level of NVIDIA Optimus support is a shit-wreck.
There are projects like Bumblebee to try to get some level of Linux support (and other advancements like DMA-BUF PRIME), but all of the attempts thus far to provide a dynamic and worthwhile NVIDIA Optimus experience under Linux have still left a lot to be desired. Some of the challenges can be attributed to current X.Org Server limitations, but NVIDIA isn't investing in addressing the problems or in other technologies (e.g. Wayland). At least on the Tegra side, they may possibly do something interesting.
Reportedly with the latest revisions to the 2012 Optimus MUX-less design, it's even worse for Linux users and causing major challenges for this vendor. The NVIDIA Linux engineers reportedly say that there still can be a MUX-ed version, but the NVIDIA hardware engineers say that's not possible with the latest hardware.
Optimus has already long been causing issues for those not after Linux pre-loads but installing Linux by themselves on laptops with an integrated Intel GPU and discrete NVIDIA GPU. AMD's technology is also in a similar sour spot, but at least it's less popular than NVIDIA's hybrid technology.