I'm from the generation which has Louise as a middle name 
I like the theory that it's a shifted generation name, so it's used to memorialise favourite relatives but not used as a first name because in the 90s Louise was strongly associated with a different generation? This is similar to the reasons we chose some of our DC's middle names, we wanted to remember relatives, but didn't particularly like the sound of the name as a first name. Looking online it seems that Louise as a first name had a popularity peak in the 1970s.
The thing of name trends is fascinating to me - especially the fact that so often people pick a name because it sounds unusual and interesting to them or they don't know many people with that name - only to find that it is then one of the top 10 names for that year. I think this probably happens because some names go in cycles, so people tend to avoid using names for babies if the name "feels like" it should belong to a grandmother or a middle aged uncle (or, indeed, every third child in your own school class). Therefore certain names get attached to a specific generation and fall out of favour for a while. But eventually, the generation who have a large amount of that name die, and because they were the great-grandparents (or older) of the current generation of new parents, that generation don't have the same association of those names as "old person names" because they don't have any memory of people in their lifetime having those names. They mainly come across them in literature for example, which makes them feel unusual and unique.
I would guess BTW this is probably where that thing comes from that so many people choose the same name while having the perception that the name is unusual - we all read the same book or watched the same film as a child, for example. Or it could simply be that in the absence of brand new names, the pool of potential names is not actually that big to begin with and if you've ruled out the "classic names" that never really go out of fashion (Jack, Elizabeth, Henry, Amelia etc) and you don't want a name associated with your own generation, your parents' generation or your grandparents' generation, then there's not that many really to pick.