Ways to ask reasons in Chinese："为什么“and“怎么”
Both "为什么" and "怎么" are used to ask for reasons, but "为什么" is usually for asking the simple reason while "怎么" is for expressing different feelings such as strange, surprised and unsatisfied.
"为什么"和"怎么"都可以用来询问原因。用“怎么”时有表示奇怪、惊讶或不太满意的感情色彩。

Examples:
1.你为什么迟到？因为路上堵车。
2.这个问题，你为什么不问问老师？
3.你知道他为什么辞职吗？

1.你怎么不试试呢？
2.他们都去吃饭了，你怎么没去？
3.昨天你怎么没来？

Question

Ways to ask reasons in Chinese："为什么“and“怎么”
Both  "为什么"  and  "怎么"   are  used  to  ask  for reasons,  but  "为什么"  is  usually  for asking  the simple  reason   while  "怎么"  is  for   expressing different feelings such as strange, surprised and unsatisfied.
"为什么"和"怎么"都可以用来询问原因。用“怎么”时有表示奇怪、惊讶或不太满意的感情色彩。

Examples:
1.你为什么迟到？      因为路上堵车。
2.这个问题，你为什么 不问问老师？
3.你知道他为什么辞职吗？

1.你怎么不试试 呢？
2.他们都去吃饭 了，你怎么没去？
3.昨天你怎么没来？

Answer

As far I'm concerned, they're virtually the same.
You can take it this way.
"为什么" is why, kind of formal.
"怎么" is how come, less formal.

But you're right to a large extent.