Because "Miss" as a title was originally developed for unmarried women during the 18th century (roughly; it didn't become a settled rule until about Austen's time). The traditional convention is that a woman who had been married but is now single again was known as Mrs Formerhusband's name.
If you are going to buy into an old-fashioned set of rules, as decribed above, then you really have to use "Mrs." If you are going to reject these old fashioned rules and go with modern practices, then you need to go Ms all the way. Trying to use "Miss" as a formerly married women or a married does not really satisfy either of these systems, and frankly I am not surprised that people are struggling a bit with it. They are probably thinking "She obviously meant to say "Ms" but her pronunciation is a bit off, so I'd better use Ms"--I know I would!
I have been Ms since I was 16 and as someone who lives outside the UK, I find it baffling how the UK continues to cling to Miss and Mrs. In most other English speaking parts of the world, they are falling out of use. For business purposes (and in most other contexts, let's face it, it's mostly first-names these days!) you use Ms unless the person has made it clear they use another title.
If I have to address someone in the UK and I don't know their title, I use Mrs for any married woman over about 65 (= my mum's friends, basically!) unless corrected, as I tend to assume that they probably prefer to use Mrs in most cases, but anyone under that age I'll use Ms as the default choice.
I really think Mrs and Miss need to die a gentle death. They serve no practical purpose, and it feels so impolite and judgmental to have to look at someone and have to start making judgments about how old they look and whether they "look" married or not.