I don't think anyone would really like to work with a truly diverse group of people. A truly diverse group would -all speak different languages -all be different genders (their origin culture, from which they immigrated last week, has some alternate view of gender) -all have different views on what's normal and what's tabboo -all have vastly different religious/quasi-religious views
The hip, trendy open-plan office would be an open war zone between the eunuch sun-god worshipper and the revivalist post-truth anti-christianite on the first day.
diversity is generally considered a good thing in evolution. i dont know if that applies to concepts like the tech industry, but maybe it does.
i am not against anyone and think the universe of software is a big one with a ton of room.
software is not created in a vacuum, nor is any other art or form. i want us all doing it, not just a bunch of people like me in every way.
-to signal virtue for PR purposes -to expand hiring pools and thus drive down wages -to get more sexual partners -to create leverage points for internal politics (e.g. opportunities to accuse political opponents of sexism)
Perhaps you were actually asking about bad outcomes, not reasons:
-women are rationally perceived as less competent after they were given preference during hiring -women who would have been happier in another field are encouraged into tech where they are less happy than they would have been in medicine, or law, or marketing, or at home -women who aren't up to the task are admitted; the mismatch sets them up for failure where they could have succeeded and had better personal outcomes in another job
Both my above lists are offhand samples.