As a reader of manga and comics I'll say this: Comics are at a disadvantage due to their very nature. You have God knows how many writers who've worked on a character for a particular run. As such, the quality can vary wildly as a result. Tone and direction are also affected as well. This is why you can have people who won't read comics about a particular character for some time. I remember having a buddy who refused to read any Spider-Man stuff after One More Day. Or you can have opinions like 'Only Jim Starlin can write Thanos'.
Manga, on the hand, is usually one person's vision. Yes, quality can improve or dip but you know it's coming from a particular person. In addition, a lot of comics just go on, forever. And once you've read enough comics, you'll know the trends, the gimmicks, the everything. Most manga, should they be allowed to complete themselves, are contained stories with twists and turns that matter. Yes, they may get sequels, prequels or spin-offs, but they are their own tale with a beginning, middle and end. So, if you have a really good manga and it stays good, well you're opinion will probably remain positive about it.
With a comic, however, that opinion will often change. One writer may do an extremely good storyline but then the next comes in a ruins what their predecessor set up. Worse yet, that bad storyline becomes part of the character's history. Enough of such things can sour people's opinion of the character (*cough*Captain Marvel*cough*). And because the character's ongoing story doesn't end, the chances of a reader getting closure are...slim.
To sum: Manga are usually self contained stories that you can easily form an opinion about. Comics are every changing rollacoasters that can tire you out after a while and have a tendency to yank your chain.
