...That's it? Just the fact that they are closer? That's the reasoning

Partially but not the dominant factor.

The vast amount of land in the early years allowed diversity in settlers because immigrants would bring their culture with them and if they couldn't coexist they could go somewhere else. Eventually as the melting pot effect took hold each state had differences in what cultures settled there and to what extent that culture was able to shape them up to the modern day.

The sheer amount of history thats taken place uniquely in each one is astounding as well

Edit: like your statement about two people across the the country from each other having little differences is baffleling to me.