As it stands, the two "styles" have very different reasons for being generic trash. Granted, you probably just meant the Western example with the Japanese cover being an example of something far better, but fuck it, let's roll.
For the Western one, it's seems to have arisen in the wake of the Disney Princess shit taking off, the art being similarly-innocuous yet somewhat-off-looking, and such copycat art tries to copy the bright colors and big eyes while badly attempting to be semi-realistic. Basically, it's trying to apply generic Disney aesthetics to a semi-realistic face with pastel colors, and this off mix of realism and cartooniness creates horrible fucking uncanny valley abortions 90% of the time, the other 10% just being non-horrifying but still shitty-looking. Also, it's worth noting that the character designs are extremely-generic, with the only differences being hair style/color, skin color and facial expression.
For the Eastern version, it comes from the way anime has evolved from a visual standpoint since its inception, and how the ubiquitous style for the more-generic moe shit is easily-copied. It throws all realism out the door to make something as testicle-shrivelingly cute as possible, by designing facial and body proportions around what sets off our "oh fuck it's a baby, better care for it" instinct that activates on a hair-trigger, thereby causing us to form a much stronger emotional attachment (whether that's to an actual character or simply to the concept as a whole) by pure instinct. You'll notice however, if you look past the cuteness, that the level of genericness between shown characters and between this implementation of the art style versus similar kin is comparable to the wave of Disney Princess knockoffs, again with the only differences being hair style/color and facial expression, skin color being exempt because Japan doesn't give a rat's ass what niggers want or think to the point og only putting in brown girls because someone on the team's got jungle fever.
The thing that brings these two lost cousins together, though, is the fact that they're both absurdly-cheap art styles to use with enough appeal to get the target audience to make a purchase. For the anime, it gets the Nipponese kiddies to buy it to be a pretty ice princess, while getting otakus to buy it too because they need a new waifu this week. For the Western version, little girls don't know better and the art is passable-enough that they get the gist of "pretty princesses YAY", and that's all it takes to shake their parents down for money.
The biggest difference between them, directly contrasting the above, is that the uber-generic moe style has evolved similarly to how domesticated cats did from its original form, slowly becoming the highly-efficient apex of cute-pandering exploitation of human parental instinct. It's both extremely-cute and extremely-cheap, which means you can have something that'll draw in everyone with enough cash to matter to buy it while keeping cost subterranean-low, while avoiding any kind of scrutiny or infamy like the Western version gets since almost everyone finds moe art cute by nature.
To put it another way, the moe example actually is superior from a business standpoint because the Nips have refined exploiting cuteness to harvest infinite shekels down to a science over the years, while the West is still impotently copying shitty popular media promo art and calling it good enough, since we just don't care about cartoons that much and little girls are a hilariously-easy market to pander to. That doesn't mean it's any less generic, mind you, instead this means that its flavor of generic is much more palatable to consumers and onlookers alike, as a result of everyone following successful examples until they hit a moe singularity where everything copies everything else in an infinite spiral of baby-faced cuteness with only mild differences between each iteration/peer. The Japanese found a working formula for cute art that's dirt-cheap, ubiquitous and scientifically-proven to work wonders for sales, while we're still floundering in the dark with no ideas except "THEY WANT MORE NEGROES, RIGHT? EVERYONE WANTS MORE BLACK" modern political retardation.
Doesn't mean I like the moeshit style in the slightest mind you, I actually think it looks like utter shit compared to the more experimental stuff anime did in the 80s and 90s while it was still trying to find its legs, and before otakus became a market big enough to toss nearly all originality out to market to primarily, but I gotta hand it to the short little fuckers for finding cheap-ass pandering animation's version of the infinite money printer. They've got us thoroughly-beat in that category, no question.