This is a topic that is raised every year or so one one of the email groups I belong to. I am not quite sure why this is the case though because all I ever see are the arguments for top posting.
When you hit the "Reply" button in your email program, the email you are replying to is usually copied into the window where you are going to enter your reply. Top posting is where you type your reply above the prior email that has been copied in while bottom posting is where you type your reply below it. While typing your reply below what you are replying to may initially seem to make sense because the content is then in order, there are many issues with doing so.
Bottom posting implies that the person who is going to tread your email will need to re-read all of what has come before in order to understand the context of your email. In most cases they ought to have a good enough memory to be able to have some idea of what is being referred to and if they don't then with a top posted reply they need only scroll down a little way to see what was being replied to.
Email programs open emails at the top of the email and so a top posted reply is immediately visible. whether there is a bottom posted reply you need to scroll to the bottom of the email. If email programs were expecting bottom posted replies then they'd open at the bottom of the email automatically - although then if the actual reply were longer than one screen worth you'd then have to scroll up to find the start of the reply.
Many people make a decision about emails fairly quickly. They open an email, scan the first few lines to see if it is relevant and if not they delete it. This may not apply to all of the emails they receive but it would certainly apply to those they receive as a member of an email group where not all of the emails will be relevant to them. When someone sends an email where they have bottom posted that email looks at first glance o be a duplicate of one that has already been seen and so will almost certainly be deleted without the person receiving it even realising that they didn't see the reply.
when the email being replied to makes a number of points and you want to reply separately to each point, a common way of doing so is to place your replies immediately after each point you are replying to. For those who normally top post it is common to indicate that this has been done by placing a short message to that effect at the top of the email. That way the person viewing the reply knows to scroll down to see the individual responses in context.
With the smaller monitor sizes now in use on mobile devices the chances of a bottom post being visible even if only preceded by a few lines of text from the prior post is small. The chances of a bottom post being seen when the email is first opened is smaller now than it used to be. If those people who insist on bottom posting want others to even realise that their reply is there then at the very least they need to top post a message to the effect that they have bottom posted. Others may still decide that it isn't worth the effort of scrolling down to see what they said but at least by doing that the recipients will know that there is actually a reply there and so may consider scrolling to see it.