The key part of CC-BY-SA of interest to you can be found in Section 2a5B of the legalease.
No downstream restrictions. You may not offer or impose any additional or different terms or conditions on, or apply any Effective Technological Measures to, the Licensed Material if doing so restricts exercise of the Licensed Rights by any recipient of the Licensed Material.
Quite simply said, by adding the 'NC' to your license you are imposing additional or different terms or conditions on the licensed material of which yours is a derivative work.
Similarly, this is again covered in Section 3b1 and 3b3
The Adapter’s License You apply must be a Creative Commons license with the same License Elements, this version or later, or a BY-SA Compatible License.
You may not offer or impose any additional or different terms or conditions on, or apply any Effective Technological Measures to, Adapted Material that restrict exercise of the rights granted under the Adapter's License You apply.
That is the legalese part, the plain english can be seen in the CC-BY-SA summary page:
No additional restrictions — You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits.
You may say "wait, there's that comparable license" part, and if you read the Compatible Licenses from Creative Commons you will note that BY-SA and BY-NC-SA are not listed under each other's compatible licenses which is a simple "nope" to that line of reasoning. This is again covered in the ShareAline Compatibility page from the FAQ.