I believe that people become rude when that is the only moderation tool that they have remaining in their toolbox to be able to maintain/protect the community that is there.
Look at those Yahoo Answers posts I linked above. Many of them are rude - because it is the only moderation tool that they have. Stack Overflow of old and the smaller Stack Exchange sites tend to be less rude because the tools to maintain the vision for what the site should be are sufficient.
The people who have the vision that closely aligns with the original Atwoodian idea tend to find the site (for lack of a better phrase) under attack from an endless tide of poorly asked questions. They cast down votes on them, they close vote them, they delete vote the worst of them... and it hasn't made any impact. A look at the front page and 50 new poorly asked questions are there. The tool that remains is to... curtly say the question is no good in comments.
This isn't helped at all by the belief from people who are more familiar with a zuckerbergian model of social sites where an up vote is the same as a +1 like follow user and anything negative done to a post is a judgement of the user who wrote it (rather than the content itself).
Another factor to consider is the ratio of people who are active in moderation and curation of the material (in particular, the amount of time they're willing to commit to those activities) to the amount of new content a day. When there are more people to help out, the content can get more attention and individual help. On CSEducators.SE, that ratio is very high. I'd guess its about an hour per post. On a site like Software Engineering.SE, it may be a minute per post. On Stack Overflow, it is fractions of a second per post.
With such little time available per post for the people who are trying to follow the Atwoodian vision of the site, you get: down vote, close, delete vote, and then when those run out "What have you tried?" and move on. There is not enough time to help a person posting a question unless they are active in trying to make it better. When it is clear that the person hasn't read a two minute tour, searched for questions first, or massaged the preview area of the screen with their eyeballs to realize how abysmally formatted the question is (ok, being a bit hyperbolic there) there's not much reason to invest any time into trying to make it a better post - down vote and move on, there's probably a better post somewhere... hopefully.
If its the 100th such post that one has seen and they're out of down votes, pulling a canned comment out of a user script and move on may be the most the person gets. It might be a bit terse (tangent - low context cultures tend to be a bit more on the terse side, while higher context cultures are expecting (pardon the stereotype) "Please kindly see the help page on editing. Thank you." in their interactions - this friction of expectation might be driving a bit of the negative perspective).
And after that, people consider Stack Overflow to be negative. Its not negative
So yes... there is a middle ground. A smaller site where there is a surplus of time and ability for moderators to do their moderation. Ideally one where new users have spent enough time on the site to understand the model for the site and its vision and goals.
You get the same thing on mailing lists. People talking about their favorite cartoon on Linux Kernel mailing list? I'm sure you would find people being rude in trying to get that topic out of their inbox - its not what was subscribed to (and in extreme cases, kicking the person off the mailing list).
In the context of Stack Overflow, it is becoming a victim of its own success. Too many people are asking questions. The ability to moderate has been on a decline - both from people leaving the site and from (what I perceive) as a change in the vision that Stack Overflow Corporate has - from an Atwoodian vision (a library of all good programming knowledge- https://blog.codinghorror.com/introducing-stackoverflow-com/ ) to that of a Spoloskian vision (useful and interesting content relevant to programming https://www.joelonsoftware.com/2008/09/15/stack-overflow-lau... ). The tools that are left... they seem negative to people who don't subscribe to the vision of what the site should be.
Keeping this in mind - I strongly recommend reading http://www.shirky.com/writings/herecomeseverybody/group_enem...