Simple markup as a style guide and limiter
Beyond perhaps making authoring easier, there is a more subtle reason to use simple markup instead of full HTML, namely that simple markup can be deliberately limited. HTML is a very broad and general formatting language, which means that if you let people write in raw HTML they can do all sorts of odd layout things, things that do not necessarily fit into the style you want. Using a simple markup language limits people's ability to be off-the-rails creative in this way.
(While you can do the same thing by letting people write in HTML and then filtering the HTML, this is more error prone and less user friendly than a simple markup language. It's always better to just not give people a way to do something to start with than than to let them do it and then tell them 'what you did isn't acceptable'.)
In a markup language without escape hatches to HTML, content authors get exactly the style and the options that you put into the markup language and it comes out formatted exactly as you want it to be. If people have escape hatches like doing diagrams in preformatted text they are generally very glaringly obvious and ugly escape hatches, ones that are not very attractive except if people work very hard at it.
(My feeling is that the more you want a very simple and minimal presentation, the more you need a constrained markup language; people seem driven to add formatting to 'pretty things up' if they possibly can. This goes especially so if you have little control over the content authors, for example if they're random outside people who feel like writing stuff for you.)
This still works to some degree even in a good simple markup language with escapes to full HTML. Since people will usually take the easier path if it works, your simple markup becomes a subtle (usually) way of steering people towards doing things the way you want them. Sure, they could go write raw HTML, but as long as your markup works most people will stick to it. Even if people go outside your markup, your markup is still a strong hint as to how you want things to be done; it expresses a style and an approach that people will be drawn to in general.
(A corollary is that this works best for areas that your markup covers. For example, if your markup has no way to do tables at all and everyone has to do them by escaping to raw HTML, people have no guidance for how you want tables to be structured and to look. Expect anarchy and any number of tables that don't really fit in.)
The more complete, flexible, and HTML-capable a simple markup dialect is the less it intends to have this sort of editorial opinion (it may still have some simply by virtue of what it does and doesn't make easy). Conversely, a very small simple markup language is a very strong and hopefully deliberate editorial opinion on what sort of style and presentation is desired.