the game is still about the same as it was 10 years ago (same rules, graphics, ...), so some people left because they got other stuff to do.
The amount of new players showing up isn't the same because there are a LOT more possible games to play (online and not online).
The number of games has relatively increased a lot more then the number of new people using the internet. and lets not talk about the number of graphical games (like WoW) that take away players from games like this.
I think utopia isn't ever going to get as big as it used to be. the good thing about utopia was that I could play it 6 years back on my almost dead laptop. now most people have computers that are made for playing games and I can understand that most games are a lot more attractive to start playing.
Luckely I have a big imagination and I'm staring right in front of me for about five to ten minutes after performing a hit. I can see how my man slaughter the enemies army and take their land or rape the women, beheading wizards and thieves, ... aaaaaaaah, I love this game soooo much ...