There's also probably generally a lot more granularity and differences between regional dialects in periods where there wasn't the infrastructure or economic impetus for trade or travel. The printing press helps codify the written language and is vital for literacy outside of a privileged class but in terms of spoken language I think it's more of an exposure driven thing, where even if there are fundamental differences between languages in proximity the more trade and interaction the more loanwords and other things you'll see, plus the effects of a hegemonic culture establishing a lingua franca

sorry if this be incoherent, my adhd is medicated but my drunkenness is also at a medicated level