Bitcoin made it through because nobody knew that a proof-of-work based cryptocurrency would ever end up being worth several hundred million dollars or more.
Now that bitcoin exists, any competitor risks getting spanked down. Even bitcoin is at risk of getting spanked due to a 51% attack, except that it will cost more than $10 million in hardware to do that.
One alternative is using scrypt instead of sha256(), and thus GPUs aren't as effective and the one crypto currency doing that has avoided attack thus far. That currency is worth under half a million dollars of valuation at the current exchange rate and there are hardly any exchanges where it is traded so there's little financial gain from attacking it sooner rather than later. But it is still very vulnerable, in comparison to bitcoin.