There are a lot of novel series like James Bond and Twilight where each novel has a completely different title. Why do authors choose not to have a series title on their novels? Wouldn't a novel sell better if it was clearly labelled as belonging to the same series as a previous novel?
I get the impression that the book trade, certainly in the UK, regards that sort of blatant branding as a bit vulgar and only really for kids' books and licensed tie-in stuff.
Also, it's not really up to the author, unless they're already well-known and successful enough to make demands. Sales & Marketing generally make those sorts of decisions - if they'd decided they
were going to brand my novels, which are a series, like that and slapped 'WILDE/CHASE 1' above the title, then they would have done it because they thought it would increase sales in the target readership. As it is, they're 'branded' in the sense that the cover designs clearly follow a pattern, but you have to check the back to see where you are in the series. It's the same for most British serial authors - it's rare to see something like "Book 5 in the Brad Thrust series" actually on the front cover.