It’s because every supported version multiplies support burden and sometimes can prevent use of new APIs that substantially improve quality of life unless the dev is willing to turn their code into a patchwork quilt of version checks (which brings its own problems).

On Android it’s less of an issue because the SDK ships separately from the system, but there are often still substantial behavioral differences between system versions under the same SDK that can be a real pain in the rear, especially when it comes to permissions-related issues. This why it’s common for Android apps to have odd bugs or behave strangely on ancient versions of Android — while it’s easy for the dev to produce a build technically runs on a wide range of versions, properly testing against all those permutations of versions and manufacturer skins is practically speaking impossible unless you’re a sizable company that keeps a lab full of devices with CI rigged up to test against them all.