One paragraph in this story tells us everything we need to know as to why there are fewer charges. It states that UBC cases are complicated by questions of consent and intoxication. 

While alcohol and partying must also plays a roll in off campus cases of sexual assault it seems that there must be a lot more going on in off campus cases that does not appear on campus. One particular factor may be the very narrow age group involved (ie. 18 to 25) on campus.

Articles like this are not very informative. They make accusations but provide no answers.