I don't know much about the details about how it got started (i.e., how much Palmer Luckey self-funded it in the early stages) but the fact that he'd already built/sold Oculus was always going to make it easier to open doors and close funding deals (even if the politics/FB drama was a complication).
Also, building tech for the US military has some advantages. You have one customer with very high stakes and very deep pockets. Obviously it's hard to get started and you need to be building something that's uniquely useful and valuable for defence purposes, but once you achieve that, once you're in the door of the military industry, you're on pretty solid ground, and that's attractive to investors. The one hardware company i know in Australia that's doing well is also making defence tech.
It's much harder if all your potential customers (in my case, farmers) are small family businesses spread thinly in non-urban areas all over the world. Investors know that makes distrubuition much more challenging.