I have no doubt that Zerg is capable of winning games. Are they as strong or at least as versatile as the other two races? Not a chance. I refer you to
this thread for a pretty convincing look* at the state of Zerg and Terran.
With the exception of 5-roach rush and perhaps baneling bust, Zerg plays a reactionary game.
Protoss and particularly Terran have a billion different openings and the only way Zerg can survive is to pick the right one in defense.
1) Scout (in the case of walloff: Guess) what opponent could be doing.
2) Build counter.
3) If the counter was wrong, you lose. If the counter was right, you are back on equal footing (you still cannot get into a walled base).
An early game like that makes it near-impossible to win tournaments. Sure, you can win single matches, but the lack of good scouting and Zerg's weakness early game makes for a very difficult path through BO3/5 or even whole tournament environments.
(*Like others have stated, these stats are not conclusive proof. However, are you really going to tell me that this domination of Terran and to a lesser extent Protoss is caused by bad/few Zerg players? The ratio is 22T to 2Z ffs and in the case of round wins even worse: I cannot possibly imagine that there are 12 times more Terran players than Zergs entering these tournaments. Add to this that several top players have already admitted T is too versatile and Z is too weak [Dimaga, MorroW, IdrA, etc) and some have even considered switching races...)
The problem I see (from watching replays, discussions and to a lesser extent playing random for some weeks) is this:
Terran: They can techswitch without any real penalties. The reactor/techlab liftoff mechanic, as well as the unit distribution between buildings are the culprit here. You've got MMM and want Tank/Viking? Lift your barracks, land your factory and on the empty techlab, land your starport on the empty reactor, done. Build additional Factory while constructing another techlab at one of your raxes, swap when done. Want Ravens instead? Swap your Stargate to a techlab, done.
This ability, combined with Terran ALWAYS walling (so scouting being very difficult) vs Zerg makes the early game extra difficult for Zerg. Sacrifice an overlord, see two barracks+techlab? Marauders/reapers, rights? Wrong, one minute later it could be factories with techlabs and you'll have tanks eating up your counter to marauders.
Protoss is somewhat safeguarded because a core army of Stalkers is good (enough) against anything, and you only need to add relatively few units to complete that core army, Zerg is not. They have no solid jack of all trades. (other than perhaps the Hydralisk, but that is one expensive core unit)
Protoss: Are very strong in the early game, precisely the period where Zerg is weak. See: 2gate, 4warpgate, 1gate stargate, etc. To safely defend without walling off you'll need really good control on your Queen, probably some spine crawlers to back her up, etc. The counter is WAY harder than the attack itself - Any beginner can rush 12 Zealots into a Z base and do damage. The amount of money a Zerg needs to spend on early defense *just in case* sets them back significantly.
In addition there's something that I find hard to explain clearly, but I'll try.
Playing PvT, you can do a very basic scout before you ever need to put down your second gateway or cybercore. That decision can be based on scouting information. If you opted for 1gate core, you can again sac a probe up their ramp to see how many units are there to take it down and base your followup on that. It boils down to the fact that, when playing reactionary PvT, the timings JUST work out if you scout at the right times.
In ZvP, this is not the case. A zerg, due to their larva mechanic, will have to commit to certain builds before they ever get a chance to see what you are doing - you can't just build lots of Zerglings in early game and expect to have a good economy afterwards. With each of their tech buildings taking ages to build, they sort of have to base their counter on a narrowed-down-guess. They have to commit to 4-warpgate defense before I have to commit to the 4warpgate itself, to give an example, while the only thing they can see is two pylons, a gateway and a zealot. From that information, it could also be 1gate stargate.
That's definitely an issue.