This appears to be one of the most limiting factors in Lemmy right now and is not unique to Mlem. Essentially, your instance can only “see” communities that at least one user on that instance has subscribed to.
Example: I’m on the lemmy.blahaj.zone instance. I’m not subscribed to !196@lemmy.world, but someone on lemmy.blahaj.zone IS, thus it shows up in my All feed:
lemmy.directory is trying to solve this by subscribing to as many communities as they can find, serving as an “/r/All” for Lemmy as a whole. But you can’t make an account there, so the idea is you browse the directory and subscribe to any communities you like, thus making them available to other users on your instance.
Speaking as someone who lives in the US, the reason why people “prefer” it is because it’s embedded into the culture that public transportation is for poor people- temporarily embarrassed millionaires and all that. And the reason for that cultural programming is because auto manufacturers and airlines have consistently lobbied against any improvements to public transportation from the very beginning, and even had a hand in specifically designing cities to require cars. China has bullet trains that could get us safely and comfortably from one side of the country to the other in 4 hours. Most EU countries have safe, cheap, accessible public transportation that EVERYONE uses.
At the end of the day, it’s just another capitalist ploy.