What is the difference between but and because?

[Thameem]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #84 "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
(c) also
(d) because

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #84 "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: incorrect
It's not that I don't trust you because I must have some evidence of your identity.
_________________________

why here but instead becuase please explain?

[Mister Micawber]

A coordinating conjunction is needed.
_________________
Native English teacher at Mister Micawber's