Because it was completely broken and horribly unbalanced and not only did Blizzard never bother to fix it the first time around they decided to go "Eh! *shrug*" and make it worse during the expansion?
There's a reason everyone always played Zerg competitively.
* Dex_Kivuli nerd rages
Starcraft is balanced enough, that's why it's stayed popular for as long as it has. And I just checked the top 20 rankings in South Korea: 8 Terran; 6 Protoss; and 6 Zerg.
South Koreans don't count. They are robots.
As for everyone else: I spent a great deal of time working as a proctor for the CPL and WCL or whatever the fuck it was. In doing so I got to watch a great deal of matches, and got a much different fundamental understanding of how things inter-related in the game.
The game was not "balanced enough". The game was not "balanced". It was horribly lopsided, and the expansion made it worse. The Zerg were much faster, they were capable of much larger swarms of units, so the fact that their units was weaker wasn't relevant, as their units were 2/3 as strong as the Terrans and 1/2 as strong as the Protoss, but they took 1/3 the time to acquire, giving them a strength-in-numbers advantage of fairly epic proportions. Also their research took less time and their units were
considerably less resource intensive.
This game was not "balanced enough" it wasn't even balanced a little bit.
Zerg were the primary victors. No one used anyone
but the Zerg unless they had already mastered the game. This is why South Koreans don't count in this. They had mastered the game to a level where they were practically computers running the AI.
Furthermore, the game comes down to one fundamental concept "He who clicks most wins."
The proctoring software used gives a breakdown after every game of various statistics, one of the more interesting ones being number of mouseclicks. The player with 1.5-2x the mouseclicks of the opponent won the game 100% of the time.
"Skill"? Pshaw. Faster LMB finger.