Bitcoin has had a huge advantage by being the first cryptocurrency ever. Its market cap is 10x larger than the closest competitor in terms of market cap (ethereum).
It also has received significantly more press in mainstream media than other altcoins.

Since many altcoins have more advanced and complex features over bitcoin, they are even harder to understand to the ordinary person.
Try explaining "smart  contracts or the dao to someone with no knowledge of cryptocurrency and you will understand.

At some point bitcoin will be overtaken, just because it is far behind in its technology, but the question is how long that will take. No coin has gathered that kind of momentum since bitcoin.