First, forking doesn't necessarily means improving. Literally anyone with a basic knowledge of how to use a computer can fork Bitcoin.

Second, an alternative currency would have to improve very substantially upon Bitcoin to overtake it. A small incremental improvement would not be enough to overtake Bitcoin, there is just too much inertia and work involved. Perhaps a very major improvement could  but even then, the Bitcoin community might just adopt the technology and apply it to Bitcoin without adopting the alternative currency.

Litecoin only had a very minor change compared to Bitcoin (the proof-of-work algorithm) and it's not a change that makes it strictly better, it was a change more akin to a different trade off. And that's why it never took over Bitcoin.