The blank is provided after 'I was tired'. what is expected is the result of this. therefore the answer must be 'so', which denotes the result. if the blank had been given like this:
I went to bed ---------- I was tired
then, the answer would have been 'because', which denotes the reason.
When answering questions, it is better not to change the structure or syntax of sentences. try to find answers that suit the given structure. that is the expected method. isn't it so, tdol?
Yes, you are right, Julesrules. The given sentence was "I was tired so i went to bed. If we put it this way. "I was tired because I went to bed." Who gets tired of going to bed? Although, "I went to bed because i was tired," is correct.