In Mandarin usually 为什么 is used to express "why". Sometimes 怎么 is also used to mean the same thing.
What makes them different? Most grammar books I beleive introduce 为什么, but I feel many Chinese use 怎么 on WeChat. Since 怎么 also means "what" or "how", sometimes I got stuck to understand what it means on a context.
Also, is there any other words to mean "why", except them and 为何 (which is the same as 为什么 but literary, right?) and 为啥...?