Jagex is introducing less content to their games. While this isn't new, it is increasing. By"timed" content, I'm referring to content you are RS gold able to only do so much of before moving on, but generally, it offers higher rewards: weekly training activities like the circus and random events such as the evil tree, or just limited "chunks of xp" games, like Troll Invasion which can only be played once per month.
Instead of adding an extra level to the skill (like the caverns of living rock to mine) The current trend is to add something that is easily performed at different levels of skill for different xp. This has several advantages that you don't have to grind your skill, potentially decreasing the impact of bots. If you time things well you will gain cheap OSRS gold more xp per time spent training the ability. To maximize your xp gain you are obligated to do as many as possible.