Bitcoin has a large investment in its mining infrastructure, it is rather difficult for a new blockchain system to duplicate that. Some systems are serious contenders (Ethereum Classic, Ethereum Forked, Zcash, Monero, etc.)
The network effect does have a little/lot to do with it.
Bitcoin's conservative stance has also something to do with it as it promotes stability, and investors need stability.
Here is an interesting thought experiment IMO:
What if someone created a 'better' currency, but instead of creating a new blockchain from scratch, they fork Bitcoin's at some point. The mining algorithm would have to be new so that the currency is not attacked by current mining equipment.
Current bitcoin holders would have a stake in the new blockchain, as the 'old' bitcoins would be recognized on the new chain.
If this new currency offers a real advantage over the original, then people will move to it.
Of course the original bitcoin can incorporate the improvements of the new currency thereby eliminating its advantage. :)
I guess i took a scenic route here, as i wanted to show the example first.
Essentially if Bitcoin keeps evolving (and it is) no new currency should be able to take it over.