When Apple releases minor updates to iOS, it generally waits a few weeks before the new firmware is released and then subsequently blocks any downgrades to any previous versions of iOS, this is only done for a few reasons.
Just yesterday, Apple stopped signing iOS 10.3.2 only three weeks after the release of iOS 10.3.3 – an update which only contained a few minor bug fixes.
So, why does Apple restrict downgrades?
Apple releases an update, then generally 2-3 weeks after that release has settled down fine, as long as there weren’t any huge issues or bugs found, prompting people needing to downgrade quickly – Apple will then stop signing the previous release, so no users can downgrade.
Apple does this, so to keep users using the latest release as nearly all of the iOS updates contain critical security fixes and Apple has a duty to make sure if any major security concerns arise, they are dealt with effectively.
However, the ability to downgrade to older versions of iOS does effect the jailbreaking community. Apple is forever leap frogging and sometimes playing catchup with jailbreakers – making it harder and more difficult with every release for them to locate any weaknesses or holes within the OS.