My personal theory is that NoSQL happened because most people found relational database schema changes far too difficult.

I've been protected from this by the Django migrations system for over a decade at this point (that landed in Sep 2014, but predecessor South was usable for quite a few years before that point).

But if you don't have a good migrations system in place, changing the schema of your relational database (and keeping those changes synced across development machines, staging, production etc) is enormously painful - even more so for applications with a lot of production traffic.

I've seen many situations where engineers have made sub-par design decisions in order to avoid having to add or modify a column on a large, existing table for a production system.