It's important to note that with dependency checking, there's more to it than simply designing an algorithm that says, "If A depends on B, install B first". Being able to resolve dependencies also lies with the developers and how well they build their packages as well as how the users manage it. There's a reason why systems on Debian and Fedora will break if you add too many different repositories (such as the infamous ATRpms). It's also the reason why Debian in particular has such lengthy documentation on building .debs.

As a result, the reason why slackpkg doesn't resolve dependencies is the same reason why Slackware doesn't bother with it. slackpkg is just a convenient tool to grab any updates from the Slackware repository so you don't have to go to the Slackware security mailing list and download all of the updates individually.