Let us examine a case you cite in order to have "food for thoughts" so as to be able to answer your questions.
[The] Lotus Sutra  is a Mahayana Sutra written by a number of Indian
philosophers over hundreds of years.
While this is a specific belief, it is stated as a [general] fact (as attests the use of gnomic present). While the question was comparative, the answer fails to compare by taking only "one version of the story" into account and expressing it as a fact. There lies the problem.
To actually compare demands (1) putting things into perspectives, (2) not taking anyone's version as a fact, (3) but as a speech, or as a belief. Comparative analysis consists mainly in presenting or exposing what each says. It is not about what is true. It is not matter-of-factly. It could be matter-of-factly only in case the question was "I am lost! I heard the two claims... which is true?" But then, such a question would be deleted because it calls for an opinion-based answer. Similarly, an answer that is matter-of-factly and opinion-based to a "comparative religion question" ought to be deleted. A comparative model would be:
- A says this.
- A says this while B says that.
- A says this while B says that and both statements are contradictory. They debate about their disagreement in such and such a way.
From this perspective, the initial statement would be better rephrased as any of the following sentence:
- "According to Theravada, the Lotus Sutra is a Sutra written by a number of Indian philosophers over hundreds of years."
- "One the one hand, According to Theravada, the Lotus Sutra is a Sutra written by a number of Indian philosophers over hundreds of years. On the other hand, according to Mahayana, a Sutra was taught by the Buddha in Rajagriha.
In case there are contradictions, one ought to highlight that there are contradictions, and seek to clarify them by stating arguments each side establish when they debate.
Another statement was:
Sadly there is next to nothing in keeping with the original suttas,
and all you will find are contradictions.
This statement suffers three faults:
- It generalizes (nothing, all) without reference.
- The word "Sadly" makes the statement a judgment.
- "Sadly" might imply that "not keeping with the original suttas" makes it mistaken or deceptive while it is not so according to Mahayana. It is again taking only one side of things.
By fear of wordiness, I will not go through each statement that I think is questionable in the answer I down-voted. This is sufficient to show that the reasons of my down-voting did not have to do with sectarianism.
In conclusion various questions arise.
- Is Buddhism SE a site where we speak, not of facts, but of propositions regardless?
- Among the reader and the ones who answer, who is expected to be the more expert?
- What is the function of SE? (I do not believe one should have to read all the answers to one question in order to get the answer he was looking for, because it is not a forum)
I tend to think that the ones who answer should be more expert, and that if we can not demand it from anyone (as Suminda suggests) then we should not allow comparative questions.
I checked a few Q&A that were tagged "History" because historical Q&A also does not speak of Buddhism: it speaks of events that are likely to have happened (or positions that are likely to have been held, expressed, etc). A historical Q&A will not state what is fact according to Buddhism (such as "What is fruition in Buddhism?") but what History (and thus historians!) state is likely to have happened. For instance: "Did Buddha Return to Extreme Asceticism?"
I did not come across inadequate answers to questions that were tagged history. My thought as to the why is: It is because we know how to answer historical questions. Everyone knows historians debate one another... disagree with one another... come up with versions of what might have been said, etc.; express probabilities by way of using conditional, and so forth.
I believe we lack the expertise (and maybe the maturity) to answer questions of comparative religion. This is because one the one hand, we are "obsessed" with what is and what is not, and on the other hand, we are not that familiar with comparative studies. To answer adequately a question comparing two traditions, imagine how expert I must be. Is it realistic? Can we ask such an expertise of our community?