I often see this written as "Hallyu" and here it's pronounced like that, too. Why, instead of "Hanryu"?
When you speak at a normal pace, it's quite difficult to pronounce it so the ㄹ and ㄴ and some other sounds are often blended together. This goes for all words -> 한라산 (Hallasan), 왕십리 (Wangshimni), 종로구 (Jongnogu).
Note that this is not a random process, there are specific rules for this. You can either learn it from textbooks, or, pick it up with practice. It's pretty intuitive after a while.
It seems that the rules are also physiologically sensible as they apply across unrelated languages.
Latin-derived prefix in-:
Many other phonetic rules found in Korean have pretty reasonable physiological explanations as well. The key is that the rules tend towards easier pronunciation. No sound combination can resist the urge to simplify over time.