As of Dec 23 2018, https://www.reddit.com/r/askphilosophy/top/ has 85,610 readers (i.e. subscribed Redditors) and 270 users online.
Why are we less popular?
How can we be more popular?
It is good that there are multiple ways to set up question and answer communities. I view reddit as similar to facebook and twitter.
Here is a link about how reddit works for comparison: https://redditblog.com/2014/07/30/how-reddit-works-2/
I don't think stack exchanges should consider themselves in direct competition with Reddit communities (or facebook and twitter). This would mean that the first question about why we are less popular is not so important. We don't have to measure ourselves against Reddit to consider ourselves successful.
However, the second question about increasing our popularity is important. Without participants, communities become unattractive. If people find Reddit communities more valuable in terms of getting and giving answers to questions about philosophy that would significantly impact this stack exchange.
So, those of us who have been here a while need to encourage use. I imagine that would be done in two main ways:
My two cents is...
Whatever you think of this particular sub-site of stackechange there seems to be a genuine diversity of expertise, unlike reddit, which, for me, attracts a mass of content of identical quality.
That seems somewhat counter-intuitive, but also fairly evident. My use of this site is almost completely limited to asking questions, ones that I hope are challenging and creative, but are often confused. That's the risk I take, I suppose.
Why do you want the site to be more popular? It's much more likely to generate interesting questions and answers, and, if you're talking about notoriety, well it seems much higher up on google search.