I have encountered (seen or experienced) a lot of mental blocks, that stop people from learning languages. For some people, this block are temporary, but for some people, this block makes them stop learning languages all together.
The most common block that I have encountered is the 'I am bad at languages'. Normally that came from school experience, where getting a good grade ment knowing the vocabulary and grammar that the teacher wanted and making all the correct answers.
I used to be there. Declared unable to learn english in primary school. I wasn't supposed to have any talent in languages. But I am learning my fourth language right now, and even if I am not perfect in none, I can get the meaning across just fine.
There is no reason, that the predispositions can stop you from learning languages. There are millions and millions at least be-linguar people. Some of them are even more-linguar. In some country (like mine), there is exception, that at least 90% of people speak at least 2 languages. More than 50% speak at least three (but that is probably because we are really small).
But while it is possible for people to speak more than one language, that doesn't mean it is easy. Language learning is hard work. Just like learning to play a instrument, or learning to paint or learning any skill really. There is first a phase, where people suck. Not being able to get meaning across. Not understanding the word on the page. But slowly and slowly, the things improve. Until you are fluent in all four components in the language.
But the marketing of different programs make language learning seems easy, and that leads to people having high expectation. If a person things, that something is going to be easy and quick, but then is hard and takes a long time, the person quits. Simply from being discouraged.
There is also a lack of motivation. Learning languages is just like learning any other skills. If you have no reason to learn them, why do? If the language is not interesting or if there is no strong reason for using the language, then the motivation is going to be low as well. And with no motivation, the journey of the language learning will just seems too long.