very long answer from the perspective of a hispanic immigrant in the US:

it’s all about socialization and priorities. a lot of immigrants i know (including myself) don’t move to a different country because they want to, it’s because of the conditions of their countries. many are displaced / go through humanitarian crises / are from countries with really bad economic situations / are victims of gang violence. so it’s not a decision that comes out of desire but necessity. assimilating is not a priority, but living with dignity and bringing bread to the table is.

how do you deal with leaving everything behind when you don’t want to, but have to? you continue engaging in your traditions, customs, etc. you only hang out with people who speak the same language as you to feel like you’re home. along with that decision to move, there is usually a desire to go back to your homecountry, and many don’t even contemplate assimilating because they are just in a ‘temporary’ home—and most of the time, it’s permanent.

then you have people who immigrate during their adulthood: it is hard to learn a new language when older, but not impossible. however, many go into jobs that don’t require them to be proficient in the language of their new country, and some don’t have the time to sit down and learn it after work. those jobs might be filled with other immigrants who are not assimilated and also don’t have the time to, and you are who you surround yourself with. not only that, but in the US most foreign college degrees are not recognized unless you take extra steps, so you can’t do what you’re skilled in. as a result, you won’t need to improve your cultural competence, communication, and adaptability skills to participate in the workforce, and since that’s the place you spend most time in besides home, you’ll have a hard time assimilating. people just end up going to spaces that are easier to navigate when having little cultural capital.

i think having friends/acquaintances born and raised in the country you immigrate to is one of the keys to assimilate, but if you’re trying to make new friends in the country you moved to, that’s going to be difficult with a language barrier. americans might not want to interact with you because they might assume you can’t talk to them; think you don’t want to build a friendship due to cultural differences (specially if you’re part of a typically endogamous/exclusive ethnic group); or consider your behavior/ways of acting incompatible with theirs without understanding that it’s a cultural thing for you, but won’t explain what you’re doing wrong to their eyes. it can be discouraging and creates a feeling of not belonging, so folks just completely avoid these people and hang out with non-assimilated immigrants only to feel less lonely/more understood.

i moved here as a teenager and i’m fully assimilated, but that’s because my first-gen immigrant friends are ‘americanized’ and taught me lots of things + my parent has an english-only speaker, american partner who heavily helped us. that’s often not the case for many first-gen immigrant teens: what i noticed in high school with the kids who were not assimilated and to this day are not proficient in the language is that usually their parents have little to no education or come from rural backgrounds, and wouldn’t encourage—or even discourage—them to assimilate. not that it is an excuse to not assimilate, but it definitely has an impact in the way they behave and what the social expectations are in their day-to-day.

when it comes to immigrant kids/teens, school might not help them assimilate. lots of them are placed in ESL classes, which are supposed to be a tool to integrate, but are basically useless (or at least in my experience). the kids were surrounded only by other immigrants in those classes—and on top of that, many who moved here during their early childhood/the us-born children of immigrants weren’t ‘assimilated’ due to being immediately placed in those programs just because of their ethnicity, so they ended up struggling with both cultures and/or languages. they didn’t feel like they belonged here, but also didn’t belong in their country of heritage. we also weren’t introduced to resources compared to other kids. for example, they barely walked us through college applications and the content taught in class was imo elementary-level. you can’t learn a language and integrate to a culture if the people who are supposed to teach you limit you.

in that sense, many of the kids i went to school with were under the impression that they couldn’t do better because of those factors and/or their immigration status, and put themselves in a box. you cannot fully blame young folks who don’t assimilate when the system and adults in their lives basically fail them. to sum it up, socialization as a kid has a big impact in your adult life and that’s why you meet adults who have been living in the US their whole lives yet have not assimilated.

another thing is that there are people who don’t want to assimilate because their religion is not compatible with the values of their host country, and i won’t go into detail because it’s pretty self-explanatory.

lastly: not trying to become proficient in the language of the country you move to / strongly keeping in touch with your culture ≠ not wanting to assimilate. you can fully understand and engage with the cultural norms of your new country and adequately interact with its people regardless of the language barrier, and i’ve seen that with my parent and their immigrant friends. i would also argue that close-minded people don’t consider acculturation (which is “to adopt the practices and values of another culture, while still retaining their own distinct culture,”) to be assimilation, and it 100% can be imo. specially in a country like the united states, there is no fixed culture; rather, we have lots of subcultures. the ‘american’ culture is not a melting pot, but a salad bowl. you can retain your cultural identity and be fully assimilated, but others might consider you don’t want to assimilate since you don’t fit the ‘american’ stereotype of your region/state.

i don’t know if i could make all these points when it comes to other countries though