Well, I stuck with FS2004 for many reasons, the main one being that it just works, and I can spend more time simming than tweaking.
As I'm not into the Boeings & Bus's, there are far more low's and slow's for FS2004 than any other sim.
Some of the wonderful addons we have are from Milton Shupe & team, the Bill Lyons commection and many others, that are not available for other sims.
Scenery, that's not available for other sims. My home country is South Africa, so I'm using freeware Aeroworx scenery, together with suitable aircraft, Avro Shackleton, Albatros, Impala, as well as SAAF paints for Ventura's and others.
Another big feature with FS2004 is that I can have multiple installs to reduce loading time and scenery conflicts. I have 8 themed installs, ranging from Golden Wings, Alaska, a Space install, a Vintage install, a Heritage install, a Ford Tri-Motor install using all of the Late Garry Smiths aircraft, paints and scenery, including sloping runways and ramps, as well as an install for our sim club, so we all have the same scenery and aircraft. So, it's all about immersion.
Thanks to all those developers who still support FS2004.
Oh, and of course, us FS2004'ers don't have to keep on upgrading our hardware to get good simming results.. We are using a 4 year old Celeron, 4gb ram, integrated graphics, to get over 30fps in our built up Avro Shackleton, Harvard and MB326 simulators.