Why hasn't Prealgebra been made a subject yet?
Ah, great question!
In a nutshell, Prealgebra hasn't been made a subject yet because the team has been focusing on other projects.
The funny thing here is that Prealgebra has been doing so well in beta, that you can almost say that we forgot about it. It's easy to focus on things that can be improved on the site, and forget about things that are actually working.
However, that will change this week! As we all know, Prealgebra has more than exceeded the requirements for graduation, so look for it to finally make it out of beta this week!
Stay tuned! :)