Adults have a head start in knowledge
As an adult, you have a great deal of knowledge and experience. You know grammatical structures from your mother tongue. You may also have experience with other languages.
This means you recognise patterns more quickly. You can also place new words and structures more easily within what you already know.
Association makes learning faster
Adults can also connect new language to existing knowledge more quickly.
For example, if you know what income tax is, you only need to learn the word in another language. You do not need to learn the whole concept again. Associations like this help you absorb new words and ideas faster.
Speaking asks for a small identity shift
In another language, you sound different. You may also feel a little different, while of course you are still the same person.
If you normally communicate very directly, you may consciously choose to sound more polite in English, for example. When you are open to a small identity shift, you communicate more effectively in your new language.
As an adult, you do learn differently. Because of your prior knowledge and the specific way you want to use the language, you benefit from a personal programme with plenty of feedback.