Bitcoin is definitely standing on the shoulders of giants, but AFAIK none of the other pre-Bitcoin cryptocurrencies ever got implemented? With the only exception being a handful of centralized services like Liberty Reserve, though those would hardly count as cryptocurrency by today's definition.
I wouldn't say Giants because they barely made waves in the market but I won't neglect the fact that they helped in laying the path.
Bitcoin unlike the others was implemented at the right time when investors were losing faith in Banks and Government during the 2008 financial crash.
As we get more technological advanced and indoctrinated, the need of digital currency becomes prevalent.
Developing a digital currency like Bitcoin was inevitable, it just came earlier than most expected.