In my sample size of one, an attorney I talked to said that Microsoft Word was the most important software he and his colleagues used. So my guess is they're just really good with Word.
Word has pretty good revision tracking and support for footnotes which are probably the main things lawyers use more than most average people do. And remember that lawyers communicate a lot with clients, etc. too so there would be a lot of friction associated with a non-standard tool.
When I worked on an expert witness report for a big law firm we just used Word.