Why should I not top-post?
The internet is all about communication. Communication is the eliciting of a response. Effective communication is the eliciting of a desired response. Trolling is the stubborn assertion of a view that the poster knows to be obnoxious to the community of readers.
This newsgroup thrives on threaded discussions. There's no way to build a threaded discussion with top-posting on a newsgroup whose majority members conventionally use bottom-posting with selective relevant quotebacks.
Top-posting severely inhibits others from understanding the conversation, because the context of the conversation is lost.
One reason why top-posting is so disliked is that those who do it very rarely bother to snip any of the preceding post(s) - they leave masses of quoted text trailing underneath their (frequently very brief) reply. This is very wasteful of resources, especially for those with slow dial-up connections, or those who may be downloading to a laptop from a mobile phone. It is a habit which many people get into when using e-mail (when it does have some logic to support it, especially when the e-mails are taking place within a large organisation), and which they carry over into news without realising that the two mediums are very different in their nature, or that there could be any other way of doing it. The days when people were required to lurk and learn for six months before posting are long gone!
The next reason is that it is actually logical that the question should come before the answer. To quote a poster in another newsgroup:
A. No. Q. Does top-posting make sense?
A third reason is that it is often better for a reply to be interleaved with the quoted text, paragraph by paragraph, as each individual point is answered. Imagine if this were to be done with the reply before the quote, rather than after it!
A fourth reason is that, although there are some groups where top-posting is tolerated or even encouraged, it is generally discouraged, and it is good manners in any social situation to go along with what the majority are already doing when you arrive - e.g. not lighting up in a non-smoking room, or complaining about smoke in a designated smoking area, or stripping to the buff in a convent, or hugging your overcoat around you at a nudist camp.
A knock-on effect of this is that the content of a top-poster's post is less likely to be given attention than if they had posted in accordance with custom and practice in the group.