These clauses are called recitals. Taken together, they are a form of preamble.
The tradition of preambles is quite ancient. The Code of Hammurabi, for example, has a preamble. This preamble serves to establish the authority of the one enacting the laws, however, rather than his motivation.
United Nations instruments typically use a series of participial phrases as preambular paragraphs (pdf) rather than subordinate clauses, but they have the same function of establishing context and motivation.
I have not been able to locate the origin of this particular form of preamble, however. It seems to have evolved gradually from freely constructed preambles. I could not identify when this form became fixed.