Generally you can find the reason in the email, at the top of the hub in viewing mode, or in the right column in editing mode. I generally find the issue to be pretty specific (e.g. a specific untrusted link).
If they did that i.e. underlined where each of the errors were in a piece of writing then they would need to employ more staff to be editors as well as the moderators. Hubs are now checked before featuring them, in the past they relied on Hubbers to check pages on the hub hopper.
It is worth knowing that the articles here are not usually written in the style of a blog. I believe the previous comment by ritsukakunx is right, that if you do not try to use the correct grammar you will struggle to get your pages featured here. Do not let this put you off, just keep in mind that writing one article that gets approved and featured might take longer than three that do not, but which ones will be of more value to you?