1) I'd say about 10% of the reddit user base even knows what bitcoin is, and far less have any. I don't see people willing to acquire bitcoin just to participate.
2) At some point you have to host the content on a computer somewhere. Time on computers costs money. In the case of reddit, this is 1000s of dollars a day (and would be even more in a decentralized system due to the overhead of coordination). Right now the bitcoin economy is not robust enough to extract 1000s of dollars a day, and there isn't any provider that will accept bitcoin in exchange for compute at that level, or even close to it.
3) Fewer and fewer people have the ability to host content anymore, be it at home or in a datacenter. So you'd be relying on just a few people who would be willing to host content. This basically leaves you very open to an "attack" on the network by a bad actor, who could take it over with reltive ease if they were just one of a few that were hosting content.
4) Child porn. It would be way too easy for someone to put that on the network and then everyone would be at risk, further reducing the number of people willing to host (see the list of Tor exit nodes that aren't government spy nodes as an example of how few people would be willing to participate).
5) Related to number 4, the laws in different countries are different. If I host in the USA and you host in say Sweden, content that is legal for you may not be legal for me, again opening me up to liability unless I closely police the content, and unlike reddit Inc, I don't have the lawyers and common carrier protections.
Like I said, I'd love to be proven wrong, but given my experience actually running reddit, I just don't think the bitcoin ecosystem is big enough to support it today or even in the medium future.