I've covered this before, too, but it never hurts to bring it up again. The election laws are written by Republicans and Democrats, and they are not about to open things up to a system that might lead to their own demise. So, they throw every roadblock in the way of non-Republican, non-Democrat candidates that they possibly can, to bleed the funding from them, to tie them up in complex legal requirements, to do everything that they can, in short, to cause the non-party candidate to despair of the whole mess, throw up their hands, and quit.
That's one of the reasons why Trump was able to come in as a non-party vetted candidate: the man has enough money to hire the battalions of lawyers necessary to navigate through the expensive, complicated mess that our election process consists of and still remain viable. If you happen to be a member of the Old Boy Club, of course, the party will pay for all that stuff; if you're not (and Trump wasn't) then you better be pretty well-heeled, or you won't make it far, and the two major parties will make sure of that, make no mistake.
This link is quite a few years old now, but the system hasn't changed; it's the best exposition I have encountered to explain what it's like for a candidate to attempt a run for office without the backing of one of the two major parties:
The Two-Party Monopoly