(This post is in direct response to the OP, seperate from any prior discussion)
I'll say it's the ease of access.
Osu is free, small, and fast to install. After the installation is done, user are presented with this global chatbox with familiar keywords such as "uguu kawaii desu senpaiplsnoticemaiswag" etc. Result? Some feel right at home while some simply pick it up along the way.
However, should the amount of shit the user needs to do before they are eligible to participate in the community increase, the amount of cringeworthy posts would generally go down. (Environments that deal with specialty topics that are hard to pick up tend to attract less cringeworthy-post producing users, whereas easy to join, free for all communities (yes, i'm looking squarely at social media) tend to attact more)
Hence, i'll pitch a guess that the reason "easy to join" communities contain more cringeworthy posts is due to how users who just want instant, thoughtless responses to their posts of the same nature are easily committed to and retained by such environments. They aren't interested in thinking though their responses, much less waiting for quality ones. They don't find it worth to take time off in order to put themselves in an ideal envrionment for anything, they just want find a spot, put whatever's currently on their minds there and entertain themselves with immediate responses (quantity over quality, instant over delayed gratification).
That's probably why there are less cringeworthy posts on most forums compared to their ingame counterparts. Additionally, it's an alternate channel where the users go to, instead of it going to them, and the majority of cringeposters don't bother seeking out alternate channels for anything, either for discussion or for solutions (same reason as why people don't RTFM, visit the wiki, or even bother searching before complaining).
However, people who post in the first place are in no way the majority (compare online users vs users chatting, or Youtube video views vs comment count), and judging any community based on the vocal minority is more or less akin as judging any group of people by their sterotypes.
Inb4 "hurr durr hipster mainstream hater"
Its not because its mainstream. Its simply the power of popularity, ease, and being the default that retains users who are willing to bear with cringeworthy posts or post them in the first place.
Browser toolbar adware works because the same users who can't be bothered to take a minute uninstall them are the same ones who don't bother spening the extra seconds unchecking the box. They would rather get the installation faster and deal with something they didn't ask for taking up resources and space.
Similarly, the reason why alternate browsers have gained popularity (especially on mobile devices) even though stock browsers have became leaps and bounds closer to 3rd party ones was simply because the installation and conversion to alternate browsers was made easier. In fact, it made more sense to switch to an alternate browser back then compared to now. Most people consider the effort to switch way more than the benefits they would get, or simply aren't even aware of the benefits
TL;DR
Everyone starts in the cringe pool.
Those who bothered to move their arses elsewhere left, leaving behind those who like it, and those who didn't find it worth to leave.
