It’s likely two things: Simplicity of system (like Bahasa Indonesian has been mentioned) and how close the new language is to your own language. This would explain why speakers of romance and, to some extent, germanic language learners can pick up Esperanto well in a very short time.
Big however, the true measure of how quickly and thoroghly one learns is utility and need; the only languages I managed to get good at are those which I had a need, a encouraging environment, and acceptance by the culture, and the other two factors didn’t matter much in comparison.
Chinese will be easier for me if my spouse is Chinese and we live in China than if you are learning it in isolation somewhere in countryside Germany, for instance. Italian will be easier for you to learn if you have Italian heritage that you see as important to your identity and travel often to Italy.
FWIW I’m a trained applied linguist and language teacher who has learned a bunch of languages to various levels.
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