I'm not justifying or excusing it, but the pressure to serve is alive and well and some guys are not mature enough to deal with the fallout from not going on a mission. I happily served a mission, but I had a lot of experiences serving with/around Elders that had no desire to be on a mission. For the most part, they were out due to family/girlfriend/social pressure, and they caved in to the pressure rather than being honest about their desire to not serve.