Indeed, the decision was made a while back to place Baha'i om the modern category because it is a relatively modern religion.
The reason the "big three" - Juadaism, Christianity, and Islam - are placed together, is that there's a lot of cross-over between the discussions, theologically, politically, and historically.
Generally I see a lot of discussion among themselves about their connections, but don't see much chatter among Jews, Christians, or Muslims about their connections to the Baha'i faith.
I guess similar for Rastarfari - it's Abrahamic in origin, but again tends not to get much coverage from the "big three".
The Baha'i faith has roots in these faiths, but it also has roots claiming fulfillment of Buddhist ideaologies as well - I'm not sure on the details, but I should presume there are connections with Hinduism as well? So, simply put, the Baha'i faith very much stands out by itself, connecting to the other religions.
The way the categories are set up is simply for ease of reference - think of it as like a form of library classification - but because of the various cross-overs, some religions will always seem to connect to multiple areas in a way the board structure here would have difficulty coping with.
For example, Sikhism is arguably Abrahamic in its connection to Islam, but because of the tie-in with Hinduism it's in the Eastern Thought section.
Additionally, the Hare Krishna movement is ostensibly an Eastern movement - but because of it's relatively modern foundation, again it seems to make sense to have it in the Modern Religions section.
So if it helps make the issue clearer - the Modern Religions section covers religions relatively recently founded, who although may have ideological connections to other faiths, tend to be self-referenced, rather than a significant part of discussions among the original faiths.
It's not a perfect way of setting up information, but I figure someone turning to the Abrahamic area is specifically looking for information on Judaism, Christianity, or Islam, while the Modern Religions section clearer shows some of the newer religion movements over the past 100-150 years.
Hope that helps.
