Because napisała is feminine. “He wrote” (he being any masculine noun) would be napisał and “it wrote” (subject of neuter grammatical gender) would be napisało.
On a side note, the sentence in some really odd contexts would actually translate to “it wrote only one letter” in English (eg. Książka. Napisała tylko jeden list. = “A book. It wrote only one letter.”, because a book is feminine in Polish) but I cannot come up with anything grammatically correct that makes it translate to “he wrote”.
Yes. And it has actually pretty good reason – the past tense formed out of what earlier had been active past participles, which would decline just like adjectives. (And instead of he wrote it one had to say he he-who-wrote is, on napisał jest in Old Polish, or I he-who-wrote am, ja napisał jeśm in Old Polish. Those lost their old to be verb and also their adjectival flavour, but the inflection by gender remained. ;-))