Actually we do go down to noble level. Although we don't normally advertise the schedule, each of the flavours runs on a sawtooth pattern for difficulty. So for example, July's BtS game is on emperor, so August is likely to be immortal, and September is scheduled for deity. October is not decided yet but will drop back to an easy level.
The sawtooth range starts at warlords/noble/prince and goes up to immortal/deity. We always try to do it so that as far as possible, at any given time there is at least one easy (monarch or lower) and one hard (emperor or above) game running. Fiddling the schedule to meet that requirement is the thing that usually determines whether a given round will actually start at noble or prince. Warlords games are only occasional, and we also don't always go up to deity because numbers of submissions tends to drop off at the highest levels (from which we presume the highest levels aren't as popular).