"Why does this 19th Century novel have such a boring protagonist" well, for a lot of reasons, really, but one of the big ones is that you're possibly getting the protagonist and the narrator mixed up.
A lot of 19th Century literary critics had this weird hate-boner for omniscient narrators – stories would straight up get criticised as "unrealistic" on the grounds that it was unlikely anyone could have witnessed their events in the manner described, like some sort of proto-CinemaSins bullshit – so authors who didn't want to write their stories from the first-person perspective of one of the participating characters would often go to great lengths to contrive for there to be a Dude present to witness and narrate the story's events.
It's important to understand that the Dude is the viewpoint character, but not the protagonist. His function is to witness stuff, and he only directly participates in the narrative to the extent that's necessary to explain to the satisfaction of persnickety critics why he's present and how he got there. Giving him a personality would defeat the purpose!
(Though lowbrow fiction was unlikely to encounter such criticisms, the device of the elaborately justified diegetic narrator was often present there as well, and was sometimes parodied to great effect – for example, by having the story narrated by a very unlikely party, such as a sapient insect, or by a party whose continued presence is justified in increasingly comical ways.)
Is that why so many novels from before about 1940 feature a lengthy and often-extraneous prologue in which the author explains how they came to be acquainted with the protagonist of the novel, and the circumstances under which said protagonist related the entire story to the author, How I Met Your Mother-style?
That's a related device, yes; it's basically splitting the difference by retaining the conceit of the omniscient narrator, but reassuring the critics that there's no need to get their underpants in a twist because there's actually a very good explanation (which will now be set forth in excruciating detail) for how the narrator came to know all this stuff.