The Ancient Rome book list has 13 books, which are categorized simply under "Ancient Rome". Meanwhile, the Ancient Egypt book list, Ancient Greece book list and Ancient Near East book list all have well over 13 books, and are divided into categories such as political history, economy, era etc. I know that users on r/AskHistorians have their own lives and can't read and review every book, but given how Ancient Rome seems to be one of the most popular topic amongst flaired users, the list seems disappointingly small compared to other civilizations.
Someone more knowledgeable will probably jump in at some point, but the basic answer is that the book list is a totally voluntary effort made by the flaired users. So, the reason that any bit of the booklist is as it is boils down to what the flairs in that area have done with it.
So some sections are really extensive and incredibly well organized... A good example here is the section on East Asia. The section you see today is the product of a relatively recent concerted effort by a ton of flairs to reorganize and improve it. Even some folks outside of the East Asia specialization (such as myself) contributed in whatever way we could. A lot of folks dedicated some free time recommending, organizing, formatting, and recruiting other flairs to do the same. Other sections have (as yet!) not received this kind of extensive TLC and that obviously shows.