There are a lot of reasons, why learning languages is hard. If it wouldn‘t be, there people would not have spend hundreds of dollars and still have a little to show for.
First of all, learning languages is hard work. No matter what some of the products tell us, learning a language is just like learning to draw, or mathematics or learning to play an instrument. It is a discipline of itself.
That also means, that learning languages takes time. At least a months of intensive learning, or years, if taking a slower route. (add the comparison of how many people want to learn a language and how many have the time.)
Also, a person should ask themselves, if language learning is really worth the the time, that they are going to spend learning it. Learning a languages takes a dedication of hundreds of hours.
But in order to answer the upper question, there is a another one. Why learn the language? If it is for the reasons, that are connected to the outside. For example, learning a language for higher payroll. Is it really worth it? Because if it isn‘t, the motivation just isn‘t going to be there. And people need a lot of motivation to see through the long term project.
There are still a lot of people, that even knowing why they are learning the language, they are still unable to do it. I am talking about the people, that have motivation. The reason is normally wrong learning methods. Some of the products on the market are scam. Some are simply not suited for learners type of learning. Some do not have a motivating pace. Some are too quick. Some teach the things, that the learner does not need. So go to the specifics to quickly. Some to late.
But for a new language learner, navigating all of that is hard. They do not know what to pick. Pick one or more? Of the same series or different products.
It depends. For perfect pronunciation, Pimsleur. For basic speaking is Michel Thomas. But even more important it is usage of the language. Reading, listening, speaking and writing.
But while doing the four mentioned activities, the learner has to stretch himself. Getting of the comfort zone into the stretch zone (but the panic zone is only for experienced ones).
Most of the learning, of any kind really, takes place in the stretch zone. But the comfort zone is to comfortable. People tend to stay in the comfort zone. So when they hit a plateau, they stop improving.
But the main reason for that is the fear of failure. People like to think good of themselves, so they stay away from failure, not trying anything. Because of that, not learning anything.
So to summarize. The three thing that keep people from learning are motivation, wrong techniques and fear of failure.