A. Why was my proposal not agreed to?
B. Why was my proposal not agreed by you?
C. Why my proposal was not agreed to by you?
D. Why was my proposal not agreed to by you?
When a sentence is started with wh-words (Who, Which, whom, where, when, why, whose, whom, and how.) then passive voice also started with same wh-word.
Rule :
Wh-word + auxiliary verb + subject + V3 + other agents