Coupling. If any part of the program can touch a global variable, then the only way to understand how that global is used and when and why it changes is by understanding the entire program. Limiting the variable’s scope (e.g. to a module or class) makes it easier to reason about, as there’s less to learn and mentally model all at once.

Have you got Steve McConnell’s Code Complete? Read the chapter on coupling and cohesion. If not, you should. (You can nab a first edition off eBay for a few bucks.) Good for the “Why”s of software construction.