I am a long-time user and moderator on Mi Yodeya. I've observed the Christianity site (and even asked a few questions). And I was, for a time, a top user on Biblical Hermeneutics, which tried to be diverse, before ultimately leaving the site due to differences over handling doctrine. With that as background:
Robert's answer is right. And I wouldn't be writing this at all except that I think some may have dismissed his answer as being "from the outside" (he's not a heavy participant in the religion sites), so I'm here to tell you as an insider that he's right. A Stack Exchange site is not just a collection of tagged content; to be successful a site requires a community. And in order to have a community there need to be both enough commonalities and few-enough major differences. An "all religions" site will not achieve either of these goals.
You might think that all religions -- or even the smaller space of all Abrahamic religions -- have enough in common to form a community. But that commonality is, basically, a shared text (for the Abrahamic religions only) that is only a part of each of those religions. And there are some important differences in where each religion goes with that foundation, differences that fundamentally affect how questions are asked and what kinds of answers are acceptable:
Judaism has the Hebrew bible, the oral law, law codes, lots of rabbinic commentary, and interpretations built on all of that.
Christianity has its own scriptures and modifies or reinterprets the Hebrew bible in light of them. Some of its denominations have laws and formal statements of doctrine, which aren't even common across all of Christianity, let alone other religions.
Islam has the Qur'an, which (if I understand correctly) incorporates altered forms of parts of the Hebrew and Christian bibles. (Islam does not accept all of those bibles as scripture.) Islam has a system of laws (hadith), and different communities with different doctrine and laws.
What do you think is going to happen if somebody asks, on Religion.SE, what some passage from the Hebrew bible means or how to correctly observe the sabbath?
You could try to constrain questions and police answers -- take the approach used on Christianity.SE to require denomination-specific questions and bump it up a notch. I think it works for them because they share a foundation -- the concept of a community of Christians is meaningful. But I don't think it works if you bump it up a level; a community of people doing Q&A about religion isn't cohesive like that.
Ok, you might think, so we don't have a coherent community -- why not still have a single site? Because when things go wrong, they go badly wrong. If you try to bring Jews and Christians and Muslims and maybe other religions together to talk about what they have in common, there won't be a lot. So they'll talk about their differences. You'll get Christian-flavored answers on questions about the Hebrew bible, Muslim-flavored answers on questions about the gospels, people arguing about those answers in comments and chat and meta, and it makes for an unpleasant environment all around. Minorities get marginalized, people from evangelical strains feel compelled to proselytize, people get tired of all the irrelevant-to-them content driving down the signal-to-noise ratio, and some people get driven out.
There are enough challenges managing diversity within each of these communities -- because, hey, Judaism or Christianity or Islam isn't homogeneous either. Stack Exchange -- and maybe the Internet -- is not ready for a serious all-religions Q&A site.
The community is not there. The differences are too great. And the subject matter is deeply personal. Questions about God, salvation, the bible, ethics and right living, and proper observance of the law are fundamentally important to the people asking and answering them, in a way that even disputes about programming languages or operating systems cannot come close to.
I realize that this question is old and as a practical matter, the existing sites are not going to be altered now. SE doesn't yank the carpet out from under graduated sites. I found my way here from a currently-active related question.