The problem lies in the etymological root of each word.
Etymonline explains that feminization comes from feminize, which appeared in the 1650's and means:
"make feminine or womanish," from Latin femina "woman, a female" [literally "she who suckles," from PIE root *dhe(i)- "to suck")] + -ize. Related: Feminized; feminizing. Femalize (1670s, intransitive, 1709, transitive) and femininize (1868) are more rare.
Note that femininize does exist, but as you yourself discovered, it is really uncommon. Also note that the noun feminization does not come from feminine, but directly from the Latin root femina (which explains the absence of the extra syllable -in).
Whereas masculinization, according to OED, appeared later and was formed from the verb masculinize:
masculinize v. + ?ation suffix, perhaps after feminization.
Under masculinise, OED says it appeared around 1858. So masculinization appeared after feminization and may have been born through analogical derivation.
I am only offering here a minimal investigation of the matter, as I quite doubt there is written confirmation of this hypothesis. If there were, I believe OED's etymology would be less vague.