Being a bridesmaid entails a good deal of responsibility, even in the most relaxed of weddings. You have to shell out cash for a dress that will likely make your arms look like pieces of ham, in a color that adult women rarely wear, and THEN you have pep talk your friend before they go down the aisle and make the big commitment.
While the process of being a bridesmaid can also be super fun and bonding, it's an emotionally and physically demanding experience that only makes sense if the bride is a close friend or family member. Otherwise, you're paying extra cash to wear a dress you didn't pick and make small talk at rehearsal dinners with the groom's creepy cousin.
Needless to say, being a bridesmaid isn't for the mere acquaintance, and when a crucial wedding party member drops out it's usually normal to leave the arrangements as they are, or to enlist another close friend who is down for the role.