Smash is easier to understand than Dissidia. I remember it took me all of one match to get the hang of the original game. Its just understand that instead of "Reduce enemy health to 0", its "Knock person off the stage; more damage they take, easier this is" as the goal.
The other thing is with Smash, every character is exactly the same for beginners as it is for vets. By which I mean, if you pick Mario, he will always have the same moves, physics, etc. The existence of the moves never change, just how well someone can make use of them and knowledge about them, but they're still always there.
Dissidia? Not the case. Quick example:
Lets take an easy to understand character like Terra. They'll see her level 1 stuff, which is all pretty straightforward; a few projectiles, a melee move (which I believe is ground only at first), a defensive Aerial HP Attack and a sniping ground HP Attack. Ok, that's fine...
Then we come into High level play. I don't mean High Level in the sense of Experienced vs. Inexperienced, I mean literal High Level Dissidia characters, cause the game literally has levels. Suddenly Terra adds moves like Holy (COmbo), aerial/ground variations of a few attacks she didn't have before, and Meteor, and some big killer strategies she didn't have before. She'll also be facing characters with totally different moves that you can't really see, like say, low level Firion doesn't have Shield Bash, so you can attack him blindly, but higher level one will have that so yeah.
Arcade mode doesn't necessarily offset this problem either...and it hurts experienced players too. I have some character set up a specific way, but whenever I go into arcade mode, I need to double check their skill lists to know exactly what THAT one can do and where.
That's another thing that may cause things to take longer. While newbees will just go "Ok, I'll use Cecil cause I like him!", those who've played the game will actually check the characters and see just HOW Cecil is set up. This mindset applies to Smash, but again, the only difference will be the person knowing that Link is capable of x, y, and z, while someone else will only know of x and y, though they'd always have access to z, just not know about it.
IOWs, Smash, like all fighting games, is just "Choose character, you get what you picked"; Ike is Ike is always Ike is never Marth. Dissidia, a level 1 Squall is not the same as a level 50 Squall is not the same as a level 100 Squall (chose these 3 levels cause those are the 3 used for Arcade Mode purposes.)
Like I said, Dissidia just isn't tourney friendly unless everyone has a decent amount of experience at it. Game is a lot less "Pick up and play" friendly than Smash. I know at DLC4, I let a few people play the game on LOW difficulty, and they still had problems winning fights. Its not that the game is hard, its that the game has an obvious learning curve; this curve is easy to overcome if you actually spend time with the game due to concise yet descriptive tutorials, but...yeah...
I think the best comparison I can make is that Dissidia is like SO3 Versus Mode, which you may remember was a pretty train wreck experience; no one, even people who played the game, had no real clue what they were doing. Granted, you can at least check what characters HAVE in Dissidia, so its not quite as bad as going in blind ala SO3 Versus, but flipside, SO3 Versus was still "hit things, they die." Dissidia, there's the "hit things, they get weaker, you get stronger!" attacks, and the "hit things, they die" attacks. Someone might not grasp the Brave/HP system as well <_<
