The most important reason why it is the most immutable ledger is that it was invented at very first when no one except Satoshi and his crew knew about crytocurrency.
Also the idea of "proof of work" also worked well and is working fine to this day.
That's why peercoin and other POS coins has not much influence as compared to Bitcoin.
Eventually Ethereum is nearing on keeping in par with Bitcoin, but lets see what happens in 4 years from now.