My bf and I, we don't do it nearly as much as we used to (but we still do it everyday). For us it was that we are now more tired either from work or school, before we didn't have the work/school issue when we met because it was during a vacation. But for others it could also be if they're not attracted to their S/O as much. Or Scientifically, why do humans mate? To reproduce, so perhaps their body isn't ready for that and the low sex drive is a sign of that