Because there were no dedicated people writing documentation, there were developers and sales people - that was it. Non technical documentation so "how to use the system" was going to be written by sales people.
Lots of times in a company you work with what you have and finding and hiring "technical writer who has experience or wants to work with LaTeX" I would count as hard task.
I used to be on good terms with a technical writer from a company I worked for some time ago. He later became one of the very few people who were let go as a result of a merger. Also, from what I can tell, technical writer isn't a cushy job, not anywhere near even entry-level programming jobs. Also, it's usually at companies with existing and large user-base, relatively large companies... there's not a lot of jobs on the market.
I mean, I have a feeling that if a company asked a technical writer to learn LaTeX, they would probably do it, even on their own time. It's a tough market, and learning LaTeX enough to produce something isn't a huge effort (also, there are a bunch of tools that prepare the boilerplate code for documents). But I can see how sales people might not have been impressed by such prospects.
"Why sales people would need to edit it?"
Because there were no dedicated people writing documentation, there were developers and sales people - that was it. Non technical documentation so "how to use the system" was going to be written by sales people.
Lots of times in a company you work with what you have and finding and hiring "technical writer who has experience or wants to work with LaTeX" I would count as hard task.