As always, there is a reason for the madness.
Timeline threads help users of these forums understand trends in occurrences of immigration and other related events, such as spousal visa applications, naturalisation applications, etc.
If comments are allowed in such threads, they rapidly (and I mean rapidly, within weeks) lose all semblance of being timeline-only threads and become general discussion threads. They may tangentially touch on one of the timeline posts, but will be on the lines of general queries regarding that person's application. Unfortunately, with some users, when you give them an inch, they take an ell (and some times more).
That is why it is much easier not to allow even an inch and to enforce a strict no-comments-in-timelines policy.
If you want to flag a query or an error to a particular user in the forums, create an appropriate new thread, mention the person (type in "@" followed by the username of the person and then click on the username in the dropdown that appears) and they will be altered to this new thread and can respond to you there.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.