1) Unfamiliar content cannot be 100% trusted.
If it's a song that I am not familiar with, I won't risk the possibility that I'm about to accidentally play an explicit track, nor a remix or cover version, that wasn't properly tagged. There's no time for me to completely check an unfamiliar song during a live event.
This not only goes for streaming content, but also something that a guest might have downloaded on their phone. No offense, but not only do I not know you, I also don't want to be liable for your property, or have you be liable for mine if anything happens to it while your device is connected.
2) Your request might not fit the current vibe.
When the dance floor is full, I want to keep playing that kind of music until it is actually time to change things up. Believe me, that opportunity will present itself eventually. I just might not be able to properly blend & mix your request with what I'm currently playing. So, please be patient.
3) Your request might be on my client's "do not play" list.
To avoid having my clients harassed into allowing me to play what they previously prohibited, I will never admit this to a guest. I'd rather take the heat and have you feel that I'm a bad performer, than cause my employer any grief.
It's important to remember that even if you don't get to hear a particular request, I have more than enough music that you will enjoy. So, please remember to show your host some love and appreciation by dancing to the music they love, and give them the amazing memory of everybody having the best time at their event.