One of the reasons is that apps just aren't open lol. Instead of telling you why people get denied the best thing to do is explain what traits are looked for when trying to get new admins. People who are active, understand the rules, have a good relationship with the community(other admins specifically), don't start needless drama, and people who actually bother putting in actual effort when it comes to making an application. That last one is big, if you don't follow the formatting for admin apps you're gonna get denied cause it means you can't follow simple instructions.