Back in the day there were a lot of famous PK'ers, all of which abided by the honour code, thus, set a trend for everyone else. When they stopped playing and the wilderness was removed, the code was forgotten.
Then BH was brought out, the main way to make money from this was to go inside and PJ a fight in order to get some loot with a very minimal risk. Dark Bowers were amongst the main accounts used in this way, and people just became accustomed to it and accepted the risk of being PJed by people.
PK'ing was still a good way to make money, so BH was used a lot and most of the BH worlds were always full. PK'ing was still alive, that's what people are forgetting, a lot more people PK'ed when BH was around than most people think. Not as many as the old wilderness, but it certainly wasn't dying. The only thing that was dying was the honour code.
After that PvP worlds were released, with rubbish loot, masses of glitches, PJing still existed on a massive scale and it only got worse because it was so easy to use a special attack and kill someone for loot. Also, the Grand Exchange was around and a lot of people flipped and manipulated the prices in order to make bank.
That was another contrubuting factor to why the wilderness started dying. Money is now too easy to make without any risk. You can go PvM on a maxed main at either Corp, GWD, TD's, etc and make bank in a few hours, so why go in the wild and risk your items with only a slight chance of ever profitting because of the amount of risk involved?
Before the quick methods of making money meant you had to risk something. For example, Green Dragons; they were a huge source of income for legit players, making a few hundred k an hour. Which back then was a great amount of money. But because Jagex brought out new monsters and made the game so god damn easy there's just no reason to go there. Green dragons brought PK'ers, in the hope to get whips which were worth a lot of money, that caused PK'ers to bump in to each other and start fights, which made Green Drag teams, etc. Same goes for Runite Ore, and any other methods there were in the wild.
Some of you may not agree but I truly believe the main reason it's "dying" is because of new monsters and how easy it is to make money. It's so easy that you no longer have to risk your items in order to make a decent amount of cash. I'm talking about times before even Visage's existed. There were very few monsters that dropped huge amounts of cash, KBD was killed for dragon helms, Kalphite Queen was killed for Dragon Chain, which back then was over 20m. 20m was a massive amount of money and KQ was one of the only monsters that dropped any item worth anywhere near that. Even Chaos Elemental only dropped a Dragon 2h and that was seen as an amazing drop.
tl;dr. PvM killed PvP.