I think the problem is that reddit culture is virulently anti-consumerist and throw a fit every time they see an ad. I'll bet they get terrible conversion rates.
The trouble with those types is they all have very strong opinions and think most products are crap. They're not opposed to buying things, just shitty things.
On Facebook or Google, you've often got to spend at least $5-$10 per mille to be competitive in most verticals. $25 to $50 in some verticals like insurance. And it converts.
The reddit userbase just isn't as monetizable.