Many (but not all) forensic internships require in-depth background checks (especially BOP), higher hours of experience, and forensic practica experiences as part of the application process. I don't know whether that is part of the reason application numbers are lower, but it may play a role. I have also noticed that many people in our field prefer not to work with the type of clients you typically see in that type of environment (e.g. many grad students in my program automatically listed "sex offenders" when asked what type of client they feel they could not help). And as previous posters said, the environment you work in is typically far more physically hazardous than other types of sites. I have heard it said that forensic environments attract a bimodal split of the best and the worst professionals across fields - the best being the people who are very attuned to the serious ethical issues that constantly arise, are highly motivated and assertive, are passionate about SMI, the law, interdisciplinary work, etc - and the worst being people who are simply willing to work in a dangerous environment for more pay. It's not an environment where you see a whole lot of older psychologists - I'm sure both burnout and traumatic experiences contribute to higher rates of drop-out of professionals over time. That's probably more info than was necessary, but just my perspective!