In my own personal opinion, I feel there are 4 main reaons why the player count is no longer at the 45k average and peaking in the 60k numbers about 5 years ago.

1) The biggest problem area is that in my almost 8 of years of playing, the game's core mechanics (read pve) are almost exactly the same as when I started. That means 8 years of grinding the same anoms, the same missions, the same mining, the same industry, the same incursions, etc.

2) The inflation factor has a big turn of for a lot of people; i.e. too much work for to little reward. I remember when PLEX were first introduced and sat around 300mil. At the time those were still considered quite expensive, especially when you could buy a dominix hull for 50mil. That meant I could rat in a haven for 30mins each day and have enough isk to lose a BS doing silly stuff or I could buy a hand full of BCs, dozens of cruisers, etc. I had a lot more fun because I had a lot of time to actually go pew pew.

3) Cookie cutter class changes also took a lot of fun out flying different ships. Almost all hulls now follow 1-2 standard fits as nothing else really fits anymore. All of the ships lost a lot of their faction's uniqueness during tiericde and it suddenly became pretty much similar ships using slightly different weapon systems.

4) Nullsec decay and blue donoughts lead to a large number of people quitting the game as CCP dragged their feet years too long to change the nullsec mechanics. The new sov at least gets people actively playing again, but there's still room for improvement even if it was too little too late.

5)* I won't classify this one as a major issue but I feel with all of the 3rd party sites that monitor everything I feel like the game, espeically nullsec is no longer wild and dangerous. Too many areas are mapped down to the most minute details and have lost the appeal since there's nothing left to discover. More emergent gameplay that can't be calculated down to an exact science; read random generator, would be good for the game as it would add some much needed "newess" for everyone.