There are two reasons why the interlanguage fossilization happens.
Adults' learning style
Interlanguage fossilization occurs in adults learning a new language. In language learning, unlike the child, adults can't ignore about language rules and grammar. When adults study a new language, they always check the language rules or grammar. This can make learning a language much harder. In addition, adults are more likely to be influenced by their native language. In other words, the native language interferences hold adults back from improving language proficiency. Therefore, there are many adults language learners who experience the interlanguage fossilization.
"Acceptance" of a new culture can also cause interlanguage fossilization. If the learners are in new country, then they have to learn a new language. When the learners are in new cultures, they try to make it suitable themselves to new circumstances. At first, they feel nervous because there are big cultural differences. Therefore, the learners study hard. However, after time goes by, people stop studying a new language because they cannot feel nervous anymore. The learner think that they are fully adapted in the new culture and they are confident with their new language proficiency. Then, interlanguage fossilization happens.