A colon is typically used to introduce a list, explanation, or elaboration. The word "because" introduces a reason or explanation, so it is already doing the job of the colon.
Here are some correct ways to use "because":
* "I went to the store because I needed milk."
* "There are many reasons to visit this city: its history, its culture, and its beautiful scenery."
Incorrect:
* "I went to the store: because I needed milk."
This sentence is grammatically incorrect.