I would say that MMORPGs and other games in general are the reason for the population decline.
I could also say that this game is very adult oriented, and requires a level of thought, tactics and a maturity that most do not wish or simply can't reach.
I have played this game for ~15 years or so and I recall the days of 100+ people on line. I go away and come back as my play time dictates. I had a few one year breaks. Within the last few years I had quite a problem with the population of "PK is what I am here for, and RP is really just grunt and say hulk smash". I have noticed a much more friendly environment in the last 6 months or so. I don't know if that is just with my recent characters or if that is a paradigm shift... ramble ramble ... end ramble
P_N