Payment & login code
Payment, sign-in and map services retire old versions of their code. When they do, those features simply stop working.
Security fixes
Old third-party libraries stop getting security patches. The app keeps running while the risk grows.
Privacy requirements
Both stores now require privacy details about the data your app and its add-ons collect. Older apps often can't provide them.
New phones & screens
New screen shapes, sizes and system settings can leave older layouts cut off or broken.