I have exactly the same photos on my 500px and Flickr accounts (through an IFTTT setup, saves me time). However, my 500px account has ≈ 2 000 views and my Flickr account has somewhere around 10 (keep in mind that these are the total views of all my photos put together)!
What I have discovered that 500px has a 'New' photos section where you can see the most recent photos posted on the site (9gag, Imgur, Reddit and others have the same), and Flickr doesn't (at least, I haven't found it). I suspect that this is at least one part of the problem. Another one is that Flickr seems centred around people that have huge amounts of followers, and those people, I suspect, created their accounts back in the day when Flickr was just a couple of thousand people. I find it really difficult to get any traction at all on Flickr.
If you have any tips, recommendations or advice, please don't hesitate to put it here, rather than just deleting this thread as irrelevant.