Category 7 staterooms hold a maximum of three passengers. If you're searching for a stateroom for four passengers, you won't see any Category 7 staterooms.
If you have fewer than four passengers, and you still don't see Category 7 offered for the dates you selected, it's possble that they're fully booked. After all, there aren't many Category 7 staterooms in relation to the Categories 5 and 6 -- and Category 7 is the least expensive of those categories, even though the staterooms are the same except for the configuration of the verandah.