It was the only one at the time. Since then a few alternatives have been proposed but none that rival PoW, at least in terms of security.
> Blockchains are not a complicated concept. They are merely a merkle tree combined with a consensus algorithm. That's it.
I wouldn't say so, the block chain is a chain, not a tree. Actually, the use of merkle trees to represent transaction data is an implementation detail. Bitcoin would have worked fine without them (though "lightweight" clients would have suffered a bit).
There are many forms of consensus algorithms.
> Git also uses merkle trees
Blockchains are not a complicated concept. They are merely a merkle tree combined with a consensus algorithm. That's it.
People try to make these concepts way more complicated than they actually are. A lot of this crypto stuff is actually quite simple.