I noticed as well, but I don't really understand the reasons.
Shonen seems clearly more popular than Seinen (not to mention Shojo/Josei), which I cannot really understand. Imo Seinen has so much more to offer, the bandwith of genres and complexity of stories is so much larger.
To my knowledge this isn't specific for MH but a very farspread phenomena, isn't it?
Perhaps it's the lack of big names like FMA/Naruto/OP (and perhaps even Bleach

), so that people who aren't "really" interested into manga don't even bother trying. It's a pity though.
Perhaps it is the extended scope of Seinen in the end. Shonen comes straight to the point, often is "colorful" (even in black and white

), doesn't have to bother to bring on long introductions, gives easy-to-follow storylines. Anyone can get stuck to that so to say. Seinen in comparison might need a certain amount of curiosity and even patience.
Just throwing in a character like Luffy, Naruto or Ichigo doesn't happen that often in Seinen. These examples from Shonen just appear and are somewhat complete characters right off the bat, are shown a little in whatever their "everyday life" is, a bit about their characteristics, and then start on some great adventure.
In Seinen they usually are introduced differently I think. The intro of Berserk shows Guts in a state of bloodlust and perhaps even madness, it doesn't deliver an explanation for that crazy behaviour. It's something that readers have to experience later. The intro of Vinland Saga barely cares about the protagonist, who isn't that important anyways for a time. It requires a little patience to get to know these characters, probably patience which many readers don't bring for a new manga.
In Shonen people get what they expect, in Seinen it is much harder to expect the right thing of a manga before reading it. People actually don't like surprises, they often are more pleased when they get what they expected anyways. When they see "Oh, it's different?", a good amount just stops reading instead of checking for qualities.