The second stupid reason is that Martha claimed to be so intimidated by Mike's unreasonable facsimile of a writing style that she felt that he'd think she was an imbecile if she wrote to him in clear English that people could actually understand. This is nuts because it ignores their shared history of his making a paranoid and vindictive jackass of himself every time he handed her a note. Since he can't admit that he put too much stock in the opinion of a bunch of scruffy idiots who still hold him in contempt, it's easy for him to want to confuse her sharing a touching comment with her girlfriends with her sharing a good laugh at the monkey she's stringing along because he's not smart enough or self-aware enough to realize that his anger and fear of humiliation had the wrong focus.
What this means is that it made no sense for her to not write because his not being able to see what she was really doing is all the paranoia fuel a gloomy dope with a nightmare fantasy of endless humiliation based on a well-deserved self-hatred he can't admit to needed. We could see her being so clean, we could eat off of her and him going nuts making up incriminating bullshit. The problem is that we'd be asked to sympathize with the nutcase pulling the case against her out of his fat Tom Of Finland ass instead of the sitting duck antagonist because she'd be convicted by horrid wordplay about what absence really makes the heart do.