When new versions of macOS are released, such as when Yosemite was superceded by El Capitan, then Yosemite was dropped from the App Store, at a least as a new download, so only El Capitan was available at that time. The only way that you could download Yosemite was if you had previously "purchased" Yosemite, and it was already in the Purchased tab in your App Store. That used to work well as new systems replaced old
However, a number of Mac systems were dropped when Sierra was released. El Capitan was still dropped, so, even if you COULD upgrade to El Capitan, it was no longer available as a normal listing in the App Store (again, if you had previously "purchased" El Capitan, it remains available in your Purchased tab. So - how do you get El Capitan, if you have never purchased it, and your Mac is limited to El Capitan?
Apple decided to make that available, but only for those who are in that situation, through a special link to the installer, not normally accessible in the App Store.
And - no, you are limited to El Capitan. The installer for Sierra and newer will not allow you to install directly to update your system to Sierra or High Sierra.
There are
workarounds to allow you to install Sierra, etc, but it is not natively supported. Upgrading to Sierra may have limitations, which are mentioned in the article that I linked. For example, your 2008 MBPro built-in wifi card will not work, so you lose wireless.