I wasn't trying to say that it is easy. I think making any kind of enjoyable manga is hard. But as you mentioned, gag manga have a lot more freedom, which imo makes it easier to come up with gags, because you can do pretty much anything. Someone who creates a sports manga needs to know as much about the sport as possible, which includes things you don't learn while watching the sport on TV. Then the characters need to be interesting, they need to have motivations, maybe some gags in between. There needs to be character development, the character/characters need to train and improve in realistic manners (assuming it's a realistic sports manga). There need to be frequent moments that keep the interest of the reader outside of matches or during training arcs.
Looking at Haikyuu for example, there are so many elements about it that elevate it far above your average sports manga in another magazine or the sports manga in Jump that got cancelled. Even a cancelled gag manga will usually still get a good laugh out of lots of readers. But from the cancelled sports manga from Jump I've seen so far, the majority of them were pretty boring. Meanwhile I haven't really seen a gag manga that bored me out. Might be just me, but there's always usually at least one funny joke, even in the worse ones.
I agree that it gets harder the longer the series goes on, because it might get boring if jokes are repeated too often. But to me, it seems a lot harder to keep readers interested in a serious narrative over a long time. Jokes usually follow certain patterns, while you can't really use too many patterns for serious development, otherwise the manga wouldn't be its own thing anymore.
All genre are hard to make in their own way. Not one is easier than the other.
Sports manga, you need compelling characters to get attached to so that you are invested in the matches shown in the series. Character development, interaction are all important. Interesting match ups are also VERY important.
All of this applies to all other genre as well.
Battle manga, you need all of the above (aside from matches, obviously) and equally interesting antagonists. Hero Academia had a full arc for the antagonists just to make them more compelling.
Comedies often rely on their characters to make jokes land. If your character is not compelling, your jokes will just miss. There's a reason why the first few chapters of Hard Boiled Cop seemed super bland. The characters seemed hollow and didn't feel like they had any substance in them. But 4 chapters in and it's monumentally better because the characters are more defined, the jokes and settings are more defined and therefore jokes land.
Tons of gag manga fail all the time. Also, gag/comedy is a VERY subjective thing. And that automatically limits your audience based on the type of comedy you go for. Whereas, there could be multiple things about a battle manga or a sports manga that makes you read it (art, characters, setting, the sport itself etc. etc.) which helps you ignore the other shortcomings (in your eyes).
Comedy manga don't really have that freedom.
It's why more than just pure gag manga, the more successful ones in WSJ are comedy manga mixed with battle. It gives them more freedom with how they can present jokes and not just completely rely on jokes. Mashle right now is a prime example of this.
