Easy. Because silver metal has unique properties that gold doesn't have, both are valued. Litecoin doesn't have unique properties compared to Bitcoin (no, tweaking a few parameters doesn't make it unique), it won't survive. "Litecoin is silver to Bitcoin's gold" was a short-term marketing ploy for the uneducated. Doge has a chance to gain some of Bitcoin's market share, because it has been unique in marketing, that is its non-technical unique property, but only if it persists in pushing itself to new markets.

Then there is a bunch of crypto 2.0 technologies with unique to Bitcoin properties, but they are beyond the scope of this thread.