Because for all of history and across cultures and civilizations, women have never been considered equal to men (socially or legally) - with the odd exception/legend here and there, of course.
"Because you're a woman and women can't do that" - "Why?" - "Because they can't." is what it boils down to.
Now for a more interesting stroll through Canon Law:
(1) The pope is the Bishop of Rome, and is (always) already a bishop (but if not, is immediately ordained as a bishop (Can. 332)[1]).
(2) Only men who have had the sacraments of baptism and confirmation (Can. 1033)[2] can receive the sacrament of ordination (Can. 1024)[3] and thus be bishops.
This has been reaffirmed (in 1994-1995)[4][5] as being part of the deposit of faith (i.e. Dogma).
In addition, anyone who tries to confer a sacred order to a woman (and the woman in question) are latae sententiae (implicitly) excommunicated.[6]
[1] http://www.vatican.va/archive/ENG1104/_P16.HTM
[2] http://www.vatican.va/archive/ENG1104/_P3R.HTM
[3] http://www.vatican.va/archive/ENG1104/_P3P.HTM
[4] http://w2.vatican.va/content/john-paul-ii/en/apost_letters/1...
[5] http://www.ourladyswarriors.org/teach/ordisace2.htm
[6] http://www.vatican.va/roman_curia/congregations/cfaith/docum...
Of course, there's still the legend of Pope Joan[7]
[7] https://en.wikipedia.org/wiki/Pope_Joan