Found in Mary'r Room [closed]

Assuming were talking about the his-possessive and its ilk (also called the "possessive dative"), the forms with "her" and "their" were used "very rarely", so that's likely the reason.

The form with his was more popular, and would even be used (albeit rarely) with females (such as in "Mrs. Sands his maid") and things (such as in "the verse his cause"). According to the OED, his "was most commonly used after nouns ending in -s referring to masculines, perhaps because it was practically identical in sound with the regular genitive ending in -(e)s". (We know that they were pronounced the same because of people such as Gabriel Harvey who complained about it.)

The reason we use apostrophe + s is because it's a contraction of -es. In Old English -es was the strong masculine singular and strong neuter singular possessive declension, but later it would be generalized to all nouns:

In Middle English the es ending was generalised to the genitive of all strong declension nouns. By the sixteenth century, the remaining strong declension endings were generalized to all nouns. The spelling es remained, but in many words the letter e no longer represented a sound. In those words, printers often copied the French practice of substituting an apostrophe for the letter e. In later use, 's was used for all nouns where the /s/ sound was used for the possessive form, and when adding 's to a word like love the e was no longer omitted. Confusingly, the 's form was also used for plural noun forms. These were derived from the strong declension as ending in Old English. In Middle English, the spelling was changed to -es, reflecting a change in pronunciation, and extended to all cases of the plural, including the genitive. Later conventions removed the apostrophe from subjective and objective case forms and added it after the s in possessive case forms.
_{Wikipedia: English Possessive}