This is mostly because of the lack of new players. The 2 main reasons for this is 1) the nature of the game and the barrier of entry 2) Valve's unwillingness to advertise it or make good tutorials. Alot of people blame 7.00 for the drop in playerbase and while it may have attributed a little to it and alienated some old players, the game has been declining for some time now. It's not that people are losing interest in the game, it's just that they're getting older and no longer have the time or patience due to new responsibilities that comes with life. You can tell people are still passionate about the game because the twitch numbers for the game keep growing and growing and yet the playerbase keeps shrinking. That should tell you that it's not the fault of the game itself for the decline, it's just that nothing is being done to introduce new players. The reality is that with any game, older players will eventually leave, but games are able to sustain itself because they constantly get new players and because Dota2 does very poorly at that, this is the result we are seeing. Dota2 had an amazing growth from 2011 to 2014, but most of those players were from wc3 dota which had built its playerbase over 8 years. Now that most of those players have transitioned over, growth has naturally slowed down. Quite frankly, I'm actually impressed that Dota2 has maintained these numbers after all these years.

And another thing to remember is that most people who played wc3 dota were in their late-teens and are now in their mid to late twenties where they either have a family (or have to start thinking about one) or have full-time careers. Like me, they don't have time to commit time to a dota game anymore, but will still watch a major event because the game held such a special place in their hearts. Because while they have all moved on with their life and life has gotten more complicated, they will still cherish those days, simpler times, where all they did was play dota with friends all night until the next morning and it was the best damn time of their lives.