Here are my issues with the non-Google online advertising I've tried so far, and I've tried a bunch of them.
1) I sell a really niche product. You deliver a really niche audience. It is possible that my niche times your niche equals a number of interested prospects so low that it is not worth my (very expensive) time to investigate, implement, and optimize your ads. Remember, advertising on a competing website who uses AdWords costs me no time at all at the margin.
2) I only want to pay for traffic that has a chance of converting. One provider I used, who will remain nameless, charge me the same prices Google does. However, my ad (for a product which is culturally only well-suited to American customers) was getting displayed in the middle of education websites in Spain. And I was getting charged full price for that.
3) My time is very, very expensive -- I shoot for $100 an hour. Let's say it takes, oh, two hours to get everything set up for your website, including conversion tracking, writing copy to conform to your requirements, etc. And lets say that you deliver sales to me at, oh, $10 apiece.
$30 product - $10 for you = $20 for me, per sale. (We'll ignore my other marginal costs since I essentially don't have any.) $200 / $20 = 10 sales required to break even against my time. 2% conversion rate implies I usually need roughly 500 free trial downloads to generate 10 sales. 20% conversion to free trial implies I would need 2,500 clicks from interested users to generate 500 free downloads.
So to just break even with my time for setting up your service once, I need 2,500 clicks from interested users. That is if you deliver absurdly high ROI immediately right out of the gate. If you don't, then I need to analyze results and optimize my ads for you, and that again has an imputed value of $100 per hour.
Most non-Google advertising platforms I have tried require far too much of my time to justify the returns they have provided.