You can also argue that these classes are so popular because they do so well in arena and by looking at only gold rank and top 100 you already get a 94% steady picture how the balance looks like.
BM is at the top because he has 2 skill tree with which he can adjust to next to all situations
light build: double hard escapes, good vs classes that require target, have less aoe skills, good vs groups
spectral: double hm z (draw stance & normal stance), long range kd and double air borne 100-0 very good vs classes that drag matches out aka sin and fatal vs classes that only have 1 hard escape due to 3 long range defense breaking CC
Sin requires skill, alot at that, but once mastered it becomes easy just the invested time is more than other classes.
Stealth on the other hand disables 80-85% of all skills a class has, especially vs melee.
Including double escape and an uptime of 90-92% stealth, low CDs and the possibility to get out of range/to the other side of the arena at any time during stealth, because of movement speed that equals running out of combat.
Can be countered by BM due to disabling target in lightning build, untabable 100-0 combos in spectral build and def breaking skills including animation lock and no gcd on most skills; FM´s ice passive that disables 3 iframes of sin including wallbang possibility
FM Ice with passive that disables charge skills, double unlimited iframes (6s ice sheat+10s ice prison), double hard escapes, freezing an enemy even during flame wheel from destro and the 100-0 grab 4 into x wallbang combo that is not tabable even with hard escapes, because of the animation lock from pulling the enemy from afar.
Can be countered by KFM with 2 freeze on grab; BM most of the time due to no gcd with flock of blades, Sin due to Lotus and stealth; SM due to cat being able to CC independet from the summoner
SM that takes 70-80% of your HP with one air borne and the possibility to crit with cat´s pounce to get double air off and kill you in one air combo.
FM, Sin, BM and SM have a way to counter that(FM bubble, BM flock of blades, Sin with stealth and sm with petal), KFM max evasion, block and aoe def breaking CC, FM ice passive
Warden, though only with 1 escape having an immunity to CC of 85-90%, if played right, while being able to ignore damage & CC attempts and dishing out damage
All these listed are at the top.
With SM being the only out of place, because of BM´s 3rd spec and other classes being unable to counter him.
WL is out of the equation, but after 2,5years being at the top it´s nice to have breather from him.
Warden, Sin never place that far behind and BD never places on pair with BM.
That´s why it´s more objective to look at the rankings. Classes that do well are played by more people, even as alt.