Ophelia has more stuff going on than
Olivia.
Ophelia is a unique, invented name that evokes a specific character made memorable through her tragic fate (how many people could say who
Olivia was in Shakespeare?). It's much more fanciful than
Olivia, and
Olivia has a longer history as a given name.
Olivia is beautiful but also gives the bearer more options, while
Ophelia has such a strong personality it's harder for it to be flexible in terms of what kind of person could carry it off.
Olivia also has easier nickname choices. What to call an
Ophelia for short? Oph? Ophie? Feely? Nothing seems quite comfortable.
I think that's probably why
Ophelia doesn't appeal to as many people.