Larabee needs to be compatible with software in some way and if you develop for Larabee that software needs to work on other hardware with reasonably speed.

Intel would need to spend loads of money to develop software for Larabee if it differs to much from existing hardware. As a programmer you don't start to develop advanced software if there isn't a market.

If DX12 has raytracing and that works on AMD and/or nVidia GPU's. This could maybe be the entry for Larabee into the market.