Pretty easy to make up an explanation. Like so:Lizard said:More than any other version of D&D, 4e models a world which obeys the laws of narrative causality. From daily powers to magic items, you can use abilities only when it's dramatically appropriate, with no in-world explanation. Your magic items "know" when you've fought long and hard and deserve a mystical boost. There's not even a hint of game world logic to it, nor do I think there will ever be one.
Your inherent abilities (e.g. from race, class, paragon path, etc.) are all a part of you, and can be used as often as you please (within their own limitations). However, using "external" magic takes its toll, so after using a daily item ability you have to wait a while to use another. Vigorous activity (e.g. encounters) lets you recover faster from the strain, so you get one additional use every milestone. Highly experienced characters (e.g. paragon and epic tier) have a higher resistance to such strain, so they can use two or even three item abilities in short succession.
Of course, that's just a made-up justification for what the rules look like, but it works for me.