I assume this thread is about Asians not being able to pronounce the letter L properly.
It occurs for the same reason non-scottish speakers have trouble rolling their "R" the way native scottish speakers do. If a certain sound is not part of the language you learn below the age of 5 it is very hard or in some case impossible to learn it as an adult.
The same thing happens to English speakers trying to learn a language like Vietnamese. The "ng" sound in Vietnamese is very hard for a native English speaker to pronounce correctly.