How does Spanish keep gender for 1st/2nd plural person possessive determiners, while dropping it elsewhere?

Question

In Spanish, the possessive determiners nuestro and vuestro have 4 forms, depending on gender and number. However, gender variation has disappeared in other determiners, mi, tu and su (number variations survived, as mis, tus and sus).

Comparing French which preserved the gender variation for singular persons, e.g. mon (m. sg.) and ma (f. sg.), while merged genders in the plural form (there's only one mes). Italian pretty well kept them all, offering il mio, la mia, i miei and le mie, and identical variations for tuo, suo, nostro and vostro.

The unified form for plural (gender-independent) in French, mes, appears like a result of a merge. But how did Spanish "merge" the genders of possessive determiners for singular persons (i.e. mi, tu and su), while keeping it for "nosotros" and "vosotros" versions?

Answer 1

There is no mystery here.

• 1st person singular: the Latin forms meus, mea evolved somewhat irregularly to the Spanish emphatic forms mío, mía (it seems likely that there were for a while a competing form with an ascending diphthong: mió) and their plurals were formed regularly. When unstressed, as is the case always before a noun phrase, these forms lost their final vowels. This has been extremely common in Spanish historically.
• 2nd person singular: Latin tuus and tua gave old Spanish tó, túa, which apparently were reshaped by analogy with cuyo (from Latin cuius) for their emphatic forms: tuyo, tuya, with the insertion of a glide between the vowels in hiatus. The unstressed forms, as with the 1st person, lost their final vowels and finally settled on tu.

The 1st and 2nd person plural possessive pronouns, on the other hand, didn't undergo any analogical changes. They evolved regularly from the Latin forms nostrum and vostrum (plus their feminine forms in -a), giving nuestro and vuestro.