Most Helpful Guy
1. Naturally low sex drive
2. They're busy doing other things
3. Sexual repression from parents or religion or trauma
4. Low self-esteem
5. They choose that kind of sex life
6. Boring sexual partners
And probably lots more. I'd go so far to say the majority of people no matter thier looks or personality have a boring sex life and pretty much everyone has a boring sex life compared to someone else. Your exciting can easily be considered someone else's mundane.
Most Helpful Girl
Some people like their boring sex lives and are fulfilled with just that.