A lot of the decorative props are effectively unusable in other maps because they specifically meant for doomsday geometry or they're tremendously huge. Like, all the tent stuff for example.
And the hammer is one of those extremely distinctive props that immediately make players think of doomsday. Plus how many maps really need a giant hammer that's going to bash everyone that walks under it (in b4
@Crash says "All of them. All maps need this.")?
Overall just not enough usable content to make "carnival" a theme beyond what's already in doomsday imo.
Which is too bad... cause expanding that slightly into "island carnival" or "voodoo carnival" or even "amusement park" would all be awesome themes.