It's all about the ingredients, and Tax (and tastes..!)
Well, it's a good question and could be a-bit-hard-to-tell kind of question unless you are knowledgeable about the liquor regulation in Japan; a liquor tax law!
It's all about the ingredients, and the amount of malts/barley used for brewing beers. By definition, the amount of tax applied to a unit volume of beer you drink is different in proportion to how much malt/barley has been used in brewery process to produce beer. Simply put, if it contains more than 25% of malt in ingredients, it's "Beer", less than 25% ,then "Happo-shu", so and so, in terms of the price, "Beer" is expensive than "Happo-shu" because higher tax is applied to "Beer" by law! And obviously, "Beer" tastes better, bitter, and solid. than "Happo-shu" as you can tell!
What they use for additional ingredients to produce "Happo-shu" are corn, rice, and even soy beans, and portions and combinations with them depend on the brands.
Since it has already been a while since "Happo-shu" was firstly at the market, almost 10 years or so, the quality of "Happo-shu" has been greatly improved that some "Happo-shu" really taste like the authentic "Beer" now! But imagine that in a total amount of can you drink from, only a quarter or less out of it comes from pure malt and the rest are of other ingredients and stuffs, and still, I know some won't agree totally, though, still it tastes "Beer", or beery. And you can get it way cheaper than "Beer", and still you can get drunk with it. Won't it be great, will it?
My favorite "Beer" is Super Dry from Asahi Beer, definitely, and my favorite "Happo-shu"will be Kirin Tanrei from Kirin Beer.