TurboTax uses the Apple supplied developers tools to compile the software. When Apple releases a new OS they issue a new toolset to support the new features of that OS but Apple only supports 3 years with the developer tools. It is not TurboTax's decision which versions are supported by Apple. If the Apple developers tools don't support older Mac versions, TurboTax cannot.
Some 3rd party Mac software developers maintain old Apple toolsets and release several versions of their software so that you can get differed versions for different Macs, but those vendors usually only have one product and only update it once a year. TurboTax has 4 different Mac products and as the tax law changes and the IRS issues new and revised forms, usually must release an update on a bi-weekly basis - I believe there have been about 15 updates to the 2019 products since released last December. Maintaining several versions for different Macs of each product version and updates would be unmanageable and too expensive to test and support for each release would not be cost effective for a small customer base.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
