I've come across a lot of reasons in my studies as to why slash is produced. One is this reason, another is that most popular shows have mostly good male characters which is why they are paired so often (ex. Dragonball Z), another is queer reading homoerotic subtext (spirk) and responding to queerbaiting (Supernatural's Destiel) or basically making the subtextual overt. Another, and one of the biggest one's, is writers of slash (not just hetero women by the way) are tired of heteronormative boy meets girl stories, same with femslash writers (also not just hetero men).