I struggle a bit to describe why, as I'm sure some of it is romanticized by the passing of time. But it is the last time I really enjoyed it. Most of my mates I grew up with have it as their favourite as well. I guess I'd have to say it was (at the time) the best mix of detail and playability. Detail has grown immensely, but in my opinion playability has been left behind years ago.
(I could probably also draw in the lack of "negative" incidents that are now gone due to libel etc which gave it a more real feel, but that would probs be a whole other topic (loved that hidden code). I understand it, but i detracts from gameplay. I mean you can go through a season now without Joey Barton throwing a hissyfit)
As to why I play it - well I grew up with this game. I've played it since I was 14-15. Since Tommy Johnson from Derby was THE player to get once the game loaded up, when you'd play with Newcastle for Lee Clark, Robert Lee, as they'd just trashed Swindon 7-1 irl on the telly. I left it in 2007 due to what I've described earlier, but always keep an eye out for the reviews I guess. To give it another go.