Yup, and it sucks. I hate that Apple is making me choose between the OS version that I prefer on my device and security. They don't do it on macOS, and they shouldn't on iOS, either.
The iOS AppStore is a bigger revenue generator compared to the MacApp Store. I agree that Apple should at least provide security updates for prior iOS versions, however its in they interest to steer people to the newest iOS available. This is done to entice developers new and veterans to utilize the present technology to create better apps and user functionality, and to stay relevant amounts its rivials.
Remember that I agree with you on this issue, as Apple provides security updates to previous MacOS version (up to a point), however it simple economics:
New technology = New Apps to use that technology from veteran developers and new enterants.
New technology = Presently used/ established apps forces the developer to update, effectively generating a renewal of a developers account within the developers program.
New technology = May offer greater software and hardware capabilities, this may provide the perception to an end-user that iOS updates are slowing they hardware down and may be forced to upgrade to newer hardware to use that favourite app and/or hardware.
New Mac technology, Apple does not control the X86/X64 chip development and are at Intels mercy. With the A series processors being developed in-house bring greater flexibility, customization and advances to be deployed in a specific timeframe and time line.
I am uncertain, if this is done intentionally or to manage Human Resources. It’s in Apples benefit to have a streamlined team of programming departments.
Development had begun on iOS 12 shortly after WWDC 2017, same with MacOS. It had been noted a while ago that Apple has two development teams for a specific OS. This basically says that to support MacOS Client/Server, iOS iPhone/iPad, Watch OS, TVOS, HomePod OS (presently), Apple Software, future R&D projects, there are possibly 8-14 software development teams within Apple Inc. In this scenario supporting previous generation iOS security updates is of a low priority as there is not much revenue or hardware income to generate.
Third-party developers have yet to use the power of the A9 and A10/X chips and we now have A11 on the scene. The iPad Pro 2017 has been on the market for 3-4 months depending on time of announcement and time of release, and there are few case options for the 10.5” version and even fewer updated 12.9” version.
I honestly hope that Apple incorporates a software compiler in iOS 12 for programmers, maybe port XCode to iOS 12, even a lite version is better than the present non-representation.