I don't think kids are leaving at a higher rate today - the rate has been pretty consistently well over 50%, maybe even much higher, for decades.
So, to answer the question, "why do kids leave" in general, one primary reason is that the organization really offers nothing to them.
Despite the WTS lip service about the "best life ever", most kids aren't stupid. The prospect of 60 or 70 years of knocking on doors of empty houses as a pioneer, or living in squalid conditions as a missionary or "need-greater", or a lifetime of ever-increasing pressure to "do more" as a MS or elder, a lifelong boring routine of the same old talks given over and over and over and over over and over and over and over over and over and over and over over and over and over and over again, decade after decade, is a horrifying vision.
The smart ones leave. The stupid stick around and perpetuate the system.