@StoneyB's answer is completely correct, but I wanted to (literally) add more context. Here is more of the text:
My drawing was not a picture of a hat. It was a picture of a boa constrictor digesting an elephant. But since the grown-ups were not able to understand it, I made another drawing
The "but" conjoins the clauses starting "it was" and "I made". Let me re-write it, re-ordering the words and substituting "because" for the ambiguous "since":
It was a picture of a boa constrictor but I made another drawing, because [since] the grown-ups were not able to understand the first one.
That is, I think, clearer than the original, but not as good. Katherine Woods, the translator of Antoine de Saint-Exupéry's novel, chose the but-since construction as more idiomatic to the ostensible target audience, English-speaking children the age of the Prince.