The Metagame is not just about the Pokemon themselves it's also about the people who play them. Everybody involved, be it the heads or the community as a whole, are constantly learning what is viable or even good for the overall meta. Add to that the is the game changing all the time due to any additions from Gamefreak, as a result everything can and normally does go into a little bit of chaos and needs corrected.Another suspect test? What is smogon's obsession with tier shifts and banning Pokemon from tiers?
When something new is introduced, like say a new mega form, it'll tend to rise in usage due to popularity and curiosity and take the place of previous things (why Mega Pinsir dropped). Then if it turns out the new thing was good it stays up (like Keldeo) or if later discovered not as desirable it drops (Donphan) and sometimes something too good is added or dropped and needs kicked out (like Blaziken).
People change and learn all the time and so does this game and it needs adjusting every now and then.
This is not just an element of Smogon it's an element in all competitive gaming because the goal is "To achieve the most balanced game as possible." Is it perfect, no. But it's the best we have so far. And at least it isn't as bad as how Yu-Gi-Oh handles itself.