If you're wondering why this is, it's because map time limit's are handled via the server launch options.
At the moment, the settings that can be done PER MAP basis, are the one's that are generally handled within the map file itself. (Mine limit, vehicle count, airstrike cooldown, harvester dump amount and credit tick rate). Of course, there's probably a way for a mutator to change those (or just edit the map yourself, although this would include for people to download the entire file again). (My best guess.)
The case is different for the map time limit, as it is locked once the server starts up.