What changed is the hash rate first. Back in the days you could mine and have the whole client on the same machine so it wasn't necessary to have a different distinction between a dedicated miner and a node. This is still true since you can mine and manage your bitcoin node on the same machine but it's not the best way to do things any more.

Satoshi planned and developed bitcoin to resist to centralization in fact I personally don't think bitcoin has a centralization problem although we can argue about a concentration issue with modern pools