so, because
-(으)니까 expresses the reason or cause for something and corresponds to 'so' or 'because' in English.
When the verb stem ends in a vowel or ㄹ, -니까 is used, and when the verb stem ends in a consonant, -으니까 is used.
Comparison:
It expresses the reason or cause for something. Commonly used to explain the reason for one’s actions or to express to provide justification for something.