Context matters here.
First of all, every state or state-equivalent jurisdiction in the US has a system of state/territory/district highways. For the sake of completeness DC must be included even if there is only one route there.
"State parkways" are not a thing every state has, they're a thing we've created systems for only in a couple states where major statewide systems of parkways exist. So there is not a need to chase down every parkway in every state for completeness, especially given you run into the "define parkway" problem. You can't just say "any state-maintained road whose name ends in parkway" because it's arbitrary whether their names do. MD 61 happens to be named "Canal Parkway" but it could just as easily be named "Canal Road". Would you argue for creating a "Maryland Parkways" system for it if it did?
Secondly, routes with signed numbers are generally the primary focus of the project. Routes that do not have signed numbers only end up included when there is a large categorical system of them, or when they otherwise clear some threshold of importance (usually requires they be a freeway). Most roads have names, and we're not going down the rabbit hole of trying to map everything.
If there is a system MD 61 would logically fit in, it'd be a system with all the other unsigned MD routes. Now, there is some support out there for creating systems of unsigned state routes at some point in the future but it's, um, a little controversial.
Big debate/discussion of that here.