street fighter is bigger and tekken is declining right now

Tekken is still the fighting game franchise with the most copies sold and has consistently sold in good numbers in comparison to street fighter. The only really good selling street fighters are 2 and 4. Tekken 1-6 all sold over 3 million copies off of one version each. Tekken and street fighter are both on a decline honestly. 

I'd say fighting games in general are on a decline. The more recent installments of all the bigger fighting franchises did poorly.

There are a lot of elements in Brawl that are not in other fighters such as Street Fighter IV.  Here are a couple of examples.

Air game-  Smash Bros gives you the greatest amount of aeriel freedom of any fighting game (Marvel vs Capcom 3 is the only one that comes close).  In Street  Fighter 4 for instance, you can jump forward, jump backwards, or jump straight up.  Some characters have a super jump, but basically there are three types of jumps.  Once you've chosen your jump, you can use one of six attacks, most of which function very similarly.

In Smash, you have a great deal of control over your fighter in the air.  You can adjust your trajectory mid jump, air dodge, double jump (or triple as the case may be) and control the speed of your descent.  You have more maneuvarability in the air, and you generally have 5-9 attacks which typically serve unique functions.

Smash also has landing lag.  In street fighter 4, your character is able to act as soon as he or she lands.  In Smash, if you land during your attack, you will have a cetain amount of lag. This changes the way you can time your attacks in air, as well as the risk and reward of aerial attacks.

There is also the fact that ledge play is a factor of smash.  Returning to the stage is an element that is generally unique to smash, and trying to get back on the stage or prevent your opponent from reaching the stage is another element to the game and another area to be mastered.

Then of course you have different stages which all add a twist to the game.  Suppose I'm Zero Suit Samus and my opponent is Falco.  Fighting this match on Final Destination is completely different than fighting on Battlefield where I can use platforms to my advantage.  I have to learn to adapt to the stage or lose.

Basically, there is a lot to Brawl (or the other games).  There are a ton of different options, different scenarios to prepare for, and so on.  The people who say Brawl doesn't have depth to it are people who think combos=depth which couldn't be further from the truth.  Muscle memory=/= depth.

It is very hard to explain, you kinda need to get into competitive smash to understand the depth

Melee was the most technical with Lag cancelling, wave dashing etc.

But in general Smash bros has many nuances that create depth similar to many fighters out there - spacing, frame counting, hit boxes, edge guarding, stage advantages, matchups etc.

I think what sets smash bros apart is that each character has their own unique techniques and strategies

Watch this, it might be a good example

http://www.youtube.com/watch?v=CTIkAWW-iXg

Dash Attack Cancel Up - Smash

http://www.youtube.com/watch?v=0sjL8HyidZc

Hmm...if I were to grab a copy of one of the older games to practice, would you recommend Brawl or Melee?