Is the SegWit2X success due to concentration of mining power?
I feel that the existing answers here fail to highlight that while (the collection of) miners have the authority to select the order of transactions, and impose additional validity rules, they do not have the authority to determine what the rules are.

Segwit2X (in my understanding, there seem to be multiple ideas out there) seems to be a combination of a soft fork (activating SegWit) and a plan to do a hard fork later (changing some resource limits). The first is something that miners can do, but the second will only be accepted by full nodes that implement the same rules, and be ignored by everyone else.