What is the difference between but and because?

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #84 [color=blue]“Improving Verbal Skills”, question 6

It’s not that I don’t trust you I must have some evidence of your identity.

(a) although
(b) but
© also
(d) because

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #84 [color=blue]“Improving Verbal Skills”, answer 6

It’s not that I don’t trust you but I must have some evidence of your identity.

Correct answer: (b) but

Your answer was: [color=red]incorrect
It’s not that I don’t trust you because I must have some evidence of your identity.
[size=200]_________________________[/size]

why here but instead becuase please explain?

A coordinating conjunction is needed.