It's not so much that it's rules light but that it actually is well designed.
I'll give you a brief example of what I mean; most roleplaying games work like this: To hit someone you roll to hit, they roll to dodge, then if you hit, you roll damage which is a completely different set of dice and instead of trying to roll under/over, you're rolling to get as high a number as possible, then that subtracts from a total of hits. If you want to cast magic, it's a whole other system, and other powers like telepathic attacks ignore this entire ordeal. Then, if you want to do anything else, you just roll once and that's it. So you've got four or five different task resolution systems in one game. And that's without including all the fringe situational modifiers that kick in, the special rules associated with powers, stats, and skills, and on and on and on. As such, the game works like this: you roleplay, then you go to do something, everything and everyone stops, pick up the rule book, check the rules, roll and double check and so on.
I prefer games where the system doesn't stop you but rather, generates the game itself, and one rule for all task resolution, one rule for this, and that and then specific mechanics that promote a specific type of roleplaying. So if it's a PVP game it has PVP rules, if it's a horror game it has rules to promote horror and so on.
I've tested this and the simple truth is, you need only really half a dozen rules, one for all dice-rolling, and then the rest generate the adventure and get players to act in character and in genre, and it creates a great experience. But most CCGs have interchangeable rules that don't create a flavourful setting, focus incessantly on combat, and have a vast amount of rules and character traits that have no relevance in 99% of games.
A game I enjoy running is a slightly modified version of HOUSES OF THE BLOODED which is enormous fun and has essentially 4 rules: One dice-rolling system for resolving any action, Aspects to represent skills and weapons and what-not, Style points to allow players to determine the story, and a Background system to represent the character's resources. The result is a game that is extremely easy to learn and promotes a specific type of play: that is politicking barons who are engaged in trying to defeat one another. I have roleplayed this so many times, and just on Monday with a doubter, and not only does it work, people are continually amazed at how different it is. Then I run my SAGA game and they go, "That was such a SUPERHERO" game, and so on.
This might not make sense, but it's really something I keep trying to explain to people, they don't get, I sit them down and play it, and 9 times out of 10, the game is better than entire campaigns.